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It would seem that the personal goal of persons like Paul, Peter, and John was to get congregations to a point where they would reach that level of maturity. But we see Paul continuing to "shepherd" the congregation, as a kind of "long distance" elder. He is to those congregations what the "governing body" seems to be to current Witness congregations.
John, in Revelation, writes to congregations with an idea that Jesus handles each congregation directly, and that they have been "on their own" under the direction of Jesus. They have a need to recognize this direct authority of Jesus, as they make decisions locally about who/what to listen to, and who/what to avoid. (Revelation 2&3).
It seems as though the apostles and older men of the first century did indeed act like a kind of governing body (not just in Jerusalem, but in Antioch, and anywhere that Paul, Titus, Timothy, etc. might have served from). But by the time John wrote, it was important to have more reliance on the holy spirit, and the FOUNDATION of the apostles and prophets, who had already been inspired to write the Biblical guidance which came to be seen as the primary content of the scriptural canon. So you can't really get mad at people who wish to imitate these shepherds from the first century to shepherd the congregations today. But you can also see a need for a balanced view since the goal should also continue to be guiding all to rely directly on the words of the Bible already written. Teaching the congregations to be guided by the holy spirit is a more difficult concept because, to most of us, it just means following the Bible, which is our only sure and consistent source of guidance by holy spirit.

It would seem that the personal goal of persons like Paul, Peter, and John was to get congregations to a point where they would reach that level of maturity. But we see Paul continuing to "shepherd" the congregation, as a kind of "long distance" elder. He is to those congregations what the "governing body" seems to be to current Witness congregations.
John, in Revelation, writes to congregations with an idea that Jesus handles each congregation directly, and that they have been "on their own" under the direction of Jesus. They have a need to recognize this direct authority of Jesus, as they make decisions locally about who/what to listen to, and who/what to avoid. (Revelation 2&3).
It seems as though the apostles and older men of the first century did indeed act like a kind of governing body (not just in Jerusalem, but in Antioch, and anywhere that Paul, Titus, Timothy, etc. might have served from). But by the time John wrote, it was important to have more reliance on the holy spirit, and the FOUNDATION of the apostles and prophets, who had already been inspired to write the Biblical guidance which came to be seen as the primary content of the scriptural canon. So you can't really get mad at people who wish to imitate these shepherds from the first century to shepherd the congregations today. But you can also see a need for a balanced view since the goal should also continue to be guiding all to rely directly on the words of the Bible already written. Teaching the congregations to be guided by the holy spirit is a more difficult concept because, to most of us, it just means following the Bible, which is our only sure and consistent source of guidance by holy spirit.

I was only quoting you. It's something that J.B. had said about three times, and I wasn't surprised when 4J said the same thing.
Yes, you have to be very careful whom you trust on such matters. Some of these ideas are based on a belief that Paul would not have changed his instructions as the congregations matured. Paul's stance against legalism for example doesn't seem to fit his "rules" about how to identify a deserving widow. Or specific sets of rules about who can be an elder or a ministerial servant. This does not mean that the letters were not "Pauline" however, but it does mean that we should look carefully at why certain statements appear to contradict earlier letters.
The case of 2 Peter is a little more serious. It could have been taken from Peter's own writings and turned into a useful letter for the congregations based on earlier letters, but this particular letter was not accepted as Peter's own writings by several early Christian writers. Even Eusebius (300) didn't think it belonged in the canon, although it was always generally admitted that its doctrinal content was exactly what Peter would have written. There is even a good chance that it was Peter's own content, but that many Christians of the time didn't believe it because they didn't like the idea that it implied that it might be another thousand years or so from then when the parousia would actually arrive. At any rate, there is nothing significant in 2 Peter that cannot be found in other Bible books, and the part about the parousia being delayed by another thousand years has been proved true.
This is not the topic with which to discuss the canon, or authenticity, but you will see a lot of this when looking to match historical information with early Christian writings, so it can't be totally avoided.

I was only quoting you. It's something that J.B. had said about three times, and I wasn't surprised when 4J said the same thing.
Yes, you have to be very careful whom you trust on such matters. Some of these ideas are based on a belief that Paul would not have changed his instructions as the congregations matured. Paul's stance against legalism for example doesn't seem to fit his "rules" about how to identify a deserving widow. Or specific sets of rules about who can be an elder or a ministerial servant. This does not mean that the letters were not "Pauline" however, but it does mean that we should look carefully at why certain statements appear to contradict earlier letters.
The case of 2 Peter is a little more serious. It could have been taken from Peter's own writings and turned into a useful letter for the congregations based on earlier letters, but this particular letter was not accepted as Peter's own writings by several early Christian writers. Even Eusebius (300) didn't think it belonged in the canon, although it was always generally admitted that its doctrinal content was exactly what Peter would have written. There is even a good chance that it was Peter's own content, but that many Christians of the time didn't believe it because they didn't like the idea that it implied that it might be another thousand years or so from then when the parousia would actually arrive. At any rate, there is nothing significant in 2 Peter that cannot be found in other Bible books, and the part about the parousia being delayed by another thousand years has been proved true.
This is not the topic with which to discuss the canon, or authenticity, but you will see a lot of this when looking to match historical information with early Christian writings, so it can't be totally avoided.

The questions in blue were as follows:
 
There is no proof or reason to believe that the apostles themselves survived as a group until 66. The book of Acts discontinues the use of the phrase "The Twelve" very early in the narrative, and it's probably no coincidence that Acts stops referring to the apostles in Jerusalem at about the time it brings up that Herod killed one of them, James, and then immediately went after Peter. After the apostle James is killed (the brother of John) we never hear about any of the original "Twelve" again except for Peter and John. Tradition has Peter and Paul killed in the 60's, and only John surviving past the 70's. Early Christian writers like Clement, Ignatius, Polycarp and Papias, all give credence to the idea that John survived until about 96 to 100 CE. None of them mention any others as apostles except Paul and Peter who are considered to have died decades earlier.
You mentioned James, Jesus' brother, who is mentioned by Josephus, providing a context that would put his death about 62, although Epiphanius apparently thought he died at age 96, and Hegesippus is used to point to a date around 69 CE.
Jesus had only asked the apostles to stay in Jerusalem until the outpouring of the holy spirit, which would have been Pentecost, just a couple months after Jesus died. Acts speaks of them staying on a bit longer to take care of some problems of prejudice by Jews against non-Jewish Christians, and therefore making some assignments to make sure the non-Jewish Christians were treated fairly. But after mentioning the assignment of Stephen and the missionary work of Philip and his daughters, there is no more mention of the apostles. But the Jerusalem congregation was still considered to be led by "pillars" who had a lot of respect, just as Paul mentions in Galatians. Paul does not consider them to be a "governing body" however. But he did respect their decisions, even if he considered some of them as wrong. Paul directly contradicts their decision about eating meats that had been sacrificed to idols, which might even throw some question about Paul's stance on eating meats that had not been bled correctly. But Paul definitely supports the stance of the elders in Jerusalem on fornication and idolatry, of course.

The questions in blue were as follows:
 
There is no proof or reason to believe that the apostles themselves survived as a group until 66. The book of Acts discontinues the use of the phrase "The Twelve" very early in the narrative, and it's probably no coincidence that Acts stops referring to the apostles in Jerusalem at about the time it brings up that Herod killed one of them, James, and then immediately went after Peter. After the apostle James is killed (the brother of John) we never hear about any of the original "Twelve" again except for Peter and John. Tradition has Peter and Paul killed in the 60's, and only John surviving past the 70's. Early Christian writers like Clement, Ignatius, Polycarp and Papias, all give credence to the idea that John survived until about 96 to 100 CE. None of them mention any others as apostles except Paul and Peter who are considered to have died decades earlier.
You mentioned James, Jesus' brother, who is mentioned by Josephus, providing a context that would put his death about 62, although Epiphanius apparently thought he died at age 96, and Hegesippus is used to point to a date around 69 CE.
Jesus had only asked the apostles to stay in Jerusalem until the outpouring of the holy spirit, which would have been Pentecost, just a couple months after Jesus died. Acts speaks of them staying on a bit longer to take care of some problems of prejudice by Jews against non-Jewish Christians, and therefore making some assignments to make sure the non-Jewish Christians were treated fairly. But after mentioning the assignment of Stephen and the missionary work of Philip and his daughters, there is no more mention of the apostles. But the Jerusalem congregation was still considered to be led by "pillars" who had a lot of respect, just as Paul mentions in Galatians. Paul does not consider them to be a "governing body" however. But he did respect their decisions, even if he considered some of them as wrong. Paul directly contradicts their decision about eating meats that had been sacrificed to idols, which might even throw some question about Paul's stance on eating meats that had not been bled correctly. But Paul definitely supports the stance of the elders in Jerusalem on fornication and idolatry, of course.

Naturally, if it's something that might be historical but isn't found in the Bible itself, the first place you'd look is in the writings of contemporaries of first century Christians like
Josephus, Pliny the Elder, Pliny the Younger You might also try persons who referenced famous persons from the first century in works that referenced say Philo or Gamaliel, etc., even though they died before the Temple was destroyed. The same would go for biographies of the Romans or their military exploits that might reference:
Herod Agrippa Cestius Gallus Gessius Florus Nero, Galba, Otho, Vitellius, and Vespasian. (the 5 Roman Emperors between 66 and 70) Then you might try to find references in or about the writings of any of the following historians who might have mentioned something by chance up to within a 100 years of the Jerusalem event:
Marcus Cluvius Rufus, (fl. 41–69), Roman history Quintus Curtius Rufus (c. 60–70), Greek history Flavius Josephus (37–100), Jewish history Dio Chrysostom (c. 40 – c. 115 AD), history of the Getae Thallus (early 2nd century AD), Roman history Gaius Cornelius Tacitus (c. 56–120), early Roman Empire Plutarch (c. 46–120), Parallel Lives of important Greeks and Romans Criton of Heraclea (fl. 100), history of the Getae and the Dacian Wars Suetonius (c. 69 – after 122), Roman emperors up to the Flavian dynasty Appian (c. 95 – c. 165), Roman history Arrian (c. 92–175), Greek history Granius Licinianus (2nd century), Roman history Criton of Pieria (2nd century), Greek history Lucius Ampelius (c. 2nd century AD), Roman history Dio Cassius (c. 160 – after 229), Roman history Marius Maximus (c. 160 – c. 230), biographer of Roman emperors Diogenes Laërtius (fl. c. 230), history of Greek philosophers Sextus Julius Africanus (c. 160 – c. 240), early Christian The many Jewish and Jewish Christian writings to check during this period could include those with dates on them that fit the period, as seen on earlyjewishwritings.com and earlychristianwritings.com. Note that the dates are usually considered to be "scholarly" dates, not the dates that Christians assign to them when it comes to the actual NT writings. That requires a whole new discussion, but for now, those writings to check would include the following:
(Excuse the formatting issues below. I just copied them from a portion of the list at earlychristianwritings.com. It's a couple years worth of reading. I haven't completed more than a few of them, but have found nothing yet about the flight to Pella/Perea/Decapolis in any of them. The list of writings becomes much longer if you include Christian-related writings all the way up through the 300's when Eusebius and Epiphanius wrote.)
50-95 Signs Gospel 50-95 Book of Hebrews 50-120 Didache 50-140 Gospel of Thomas 50-140 Oxyrhynchus 1224 Gospel 50-150 Apocalypse of Adam 50-150 Eugnostos the Blessed 50-200 Sophia of Jesus Christ 65-80 Gospel of Mark 70-100 Epistle of James 70-120 Egerton Gospel 70-160 Gospel of Peter 70-160 Secret Mark 70-200 Fayyum Fragment 70-200 Testaments of the Twelve Patriarchs 73-200 Mara Bar Serapion 80-100 2 Thessalonians 80-100 Ephesians 80-100 Gospel of Matthew 80-110 1 Peter 80-120 Epistle of Barnabas 80-130 Gospel of Luke 80-130 Acts of the Apostles 80-140 1 Clement 80-150 Gospel of the Egyptians 80-150 Gospel of the Hebrews 80-250 Christian Sibyllines 90-95 Revelation 90-120 Gospel of John 90-120 1 John 90-120 2 John 90-120 3 John 90-120 Epistle of Jude 93 Flavius Josephus 100-150 1 Timothy 100-150 2 Timothy 100-150 Titus 100-150 Apocalypse of Peter 100-150 Secret Book of James 100-150 Preaching of Peter 100-160 Gospel of the Ebionites 100-160 Gospel of the Nazoreans 100-160 Shepherd of Hermas 100-160 2 Peter 100-200 Odes of Solomon 100-200 Gospel of Eve 100-230 Thunder, Perfect Mind 101-220 Book of Elchasai 105-115 Ignatius of Antioch 110-140 Polycarp to the Philippians 110-140 Papias 110-160 Oxyrhynchus 840 Gospel 110-160 Traditions of Matthias 111-112 Pliny the Younger 115 Suetonius 115 Tacitus 120-130 Quadratus of Athens 120-130 Apology of Aristides 120-140 Basilides 120-140 Naassene Fragment 120-160 Valentinus 120-180 Apocryphon of John 120-180 Gospel of Mary 120-180 Dialogue of the Savior 120-180 Gospel of the Savior 120-180 2nd Apocalypse of James 120-180 Trimorphic Protennoia 120-180 Gospel of Perfection 120-200 Genna Marias 130-140 Marcion 130-150 Aristo of Pella 130-160 Epiphanes On Righteousness 130-160 Ophite Diagrams 130-160 2 Clement 130-170 Gospel of Judas 130-200 Epistle of Mathetes to Diognetus 140-150 Epistula Apostolorum   140-160 Ptolemy 140-160 Isidore 140-170 Fronto 140-170 Infancy Gospel of James 140-170 Infancy Gospel of Thomas 140-180 Gospel of Truth 150-160 Martyrdom of Polycarp 150-160 Justin Martyr 150-180 Excerpts of Theodotus 150-180 Heracleon 150-200 Ascension of Isaiah 150-200 Interpretation of Knowledge 150-200 Testimony of Truth 150-200 Acts of Peter 150-200 Acts of John 150-200 Acts of Paul 150-200 Acts of Andrew 150-225 Acts of Peter and the Twelve 150-225 Book of Thomas the Contender 150-250 Paraphrase of Shem 150-250 Fifth and Sixth Books of Esra 150-300 Authoritative Teaching 150-300 Coptic Apocalypse of Paul 150-300 Prayer of the Apostle Paul 150-300 Discourse on the Eighth and Ninth 150-300 Melchizedek 150-350 Preaching of Paul 150-350 Epistle to the Laodiceans 150-350 Questions of Mary 150-350 Allogenes, the Stranger 150-350 Hypsiphrone 150-350 Valentinian Exposition 150-350 Act of Peter 150-360 Concept of Our Great Power 150-400 Acts of Pilate 150-400 Anti-Marcionite Prologues 150-400 Dialogue Between John and Jesus 160-170 Tatian's Address to the Greeks 160-180 Claudius Apollinaris 160-180 Apelles 160-180 Julius Cassianus 160-250 Octavius of Minucius Felix 161-180 Acts of Carpus 165-175 Melito of Sardis 165-175 Hegesippus 165-175 Dionysius of Corinth 165-175 Lucian of Samosata 167 Marcus Aurelius 170-175 Diatessaron 170-200 Dura-Europos Gospel Harmony 170-200 Muratorian Canon 170-200 Treatise on the Resurrection 170-220 Letter of Peter to Philip 170-230 Thought of Norea 175-180 Athenagoras of Athens 175-185 Irenaeus of Lyons 175-185 Rhodon 175-185 Theophilus of Caesarea 175-190 Galen 178 Celsus 178 Letter from Vienna and Lyons 180 Passion of the Scillitan Martyrs 180-185 Theophilus of Antioch 180-185 Acts of Apollonius

(Matthew 24:15, 16) 15 “Therefore, when you catch sight of the disgusting thing that causes desolation, as spoken about by Daniel the prophet, standing in a holy place (let the reader use discernment), 16 then let those in Ju·deʹa begin fleeing to the mountains.
(Mark 13:14) 14 “However, when you catch sight of the disgusting thing that causes desolation standing where it should not be (let the reader use discernment), then let those in Ju·deʹa begin fleeing to the mountains.
(Luke 21:20-28) 20 “However, when you see Jerusalem surrounded by encamped armies, then know that the desolating of her has drawn near. 21 Then let those in Ju·deʹa begin fleeing to the mountains, let those in the midst of her leave, and let those in the countryside not enter into her, 22 because these are days for meting out justice in order that all the things written may be fulfilled. 23 Woe to the pregnant women and those nursing a baby in those days! For there will be great distress on the land and wrath against this people. 24 And they will fall by the edge of the sword and be led captive into all the nations; and Jerusalem will be trampled on by the nations until the appointed times of the nations are fulfilled. 25 “Also, there will be signs in the sun and moon and stars, and on the earth anguish of nations not knowing the way out because of the roaring of the sea and its agitation. 26 People will become faint out of fear and expectation of the things coming upon the inhabited earth, for the powers of the heavens will be shaken. 27 And then they will see the Son of man coming in a cloud with power and great glory. 28 But as these things start to occur, stand up straight and lift up your heads, because your deliverance is getting near.”
The words of Mark and Matthew implied that the Romans would have come right up to the Temple to defile it, and that this was the time to leave as quickly as possible. We know from Josephus that Jews read Daniel's "abomination of desolation" to be based on Antiochus IV, who: according to common knowledge had done as follows, 200 years earlier:
In 168 BC, the Greek king Antiochus IV Epiphanes invaded Jerusalem and captured the city. He marched into the Jewish temple, erected a statue of the Greek god Zeus, and sacrificed a pig on the altar of incense. This provoked a revolt in Judea as the Jews fought to remove Antiochus’ sacrilege from the temple. https://www.ligonier.org/learn/devotionals/abomination-temple/
But Titus did not do anything akin to this in 66. Although in 70, he did. You can see almost direct evidence of this today by looking at the Arch of Titus.
In that year, the Roman general Titus invaded Jerusalem to crush a Jewish revolt, entered the temple, had the building destroyed, and carried off the lampstand and other temple artifacts to Rome.
So, it seems likely that it was not specifically anything in 66 (in Jesus' prophecy) that would have triggered a fleeing to Pella, nor does anyone who believed in a Pella flight actually time it to 66. Cestius Gallus did plunder the Temple [funds] and it resulted in a counter-attack by the Jews that was mostly successful. So this was an excellent time to leave, and both Jews and Romans got out of the city at that time. Wikipedia says:
The Roman governor, Gessius Florus, responded by plundering the Second Temple, claiming the money was for the Emperor, and the next day launching a raid on the city, arresting numerous senior Jewish figures. This prompted a wider, large-scale rebellion and the Roman military garrison of Judaea was quickly overrun by the rebels, while the pro-Roman king Herod Agrippa II, together with Roman officials, fled Jerusalem. As it became clear the rebellion was getting out of control, Cestius Gallus, the legate of Syria, brought in the Syrian army, based on Legion XII Fulminata and reinforced by auxiliary troops, to restore order and quell the revolt. Despite initial advances and the conquest of Jaffa, the Syrian Legion was ambushed and defeated by Jewish rebels at the Battle of Beth Horon with 6,000 Romans massacred and the Legion's aquila lost.
But at that point, Christians would have many months of opportunities to leave the city between 66 and the actual surrounding of Jerusalem in 70. 66 fits some of what Jesus said, but it was not a case so desperate that one would be unable to even grab a coat from inside your house. That was more like the situation just before Passover in 70. This is probably why Eusebius, who had read both Josephus and knew the Bible very well, believed the fleeing to Pella to be based on an angelic revelation.

Using the quotes extracted from Eusebius and Epaphanius in a Wiki article https://en.wikipedia.org/wiki/Flight_to_Pella
It might be interesting to note that the impetus to leave Jerusalem and go to Pella was not specifically credited to Jesus' prophecy in Matthew/Mark/Luke, but to an angel, or a specific oracle/revelation/prophecy given just before the war. This would put it on par with the prophecy of Agabus (Acts 11:27, 28) . . .In those days prophets came down from Jerusalem to Antioch. 28 One of them named Agʹa·bus stood up and foretold through the spirit that a great famine was about to come on the entire inhabited earth, which, in fact, did take place in the time of Claudius.
This is a curious report then by Eusebius, that he doesn't tie it to Matthew 24, or Luke 21, for example. (Epiphanius may have "corrected" this nearly 100 years after Eusebius, when he credited the flight to Jesus warning about the city being surrounded.)
The idea that the command only went to those in the city who were worthy, might also imply that there were reports that some [less worthy] Christians had died in Jerusalem's destruction.   Epaphanius had referenced Jesus' prophecy in his book Panarion, but in "Weights and Measures" he pretty much agreed with Eusebius:  

Unfortunately, this has been going on even from the time that the scriptures were written, or at least from very shortly after the NT was completed. This means that even the very idea that there had been an escape to Pella might just be from persons with their own agenda.
The best evidence that comes down to us is from Eusebius of Caesarea and Epiphanius of Salamis. Eusebius wrote his "Ecclesiastical History" (Church History) between about 300 and 325. Epiphanius would have written "Panarion" around 375. We don't know what, if anything, was written on this topic between 70 CE and 300 CE.
So this might be a little like someone just now trying to make a story about what direction we believe small bands of native Americans (Indians) ran to in 1775 in upstate NY when Ethan Allen and Benedict Arnold were capturing the guns at the British Fort Ticonderoga (which would just precede a siege of Boston and the building of a fortification of stakes around parts of that city).
The "American Indians" were not a big part of that story, so there is very little written down about what they did. If someone came up with a new story about it 200 to 250 years later, we might not put much trust in it. But, then again, we might assume that there were some verbal or even written records that could be gathered up from various families in the area, and that there was some truth to such a story.
We have some evidence that the apostles generally stayed in Jerusalem, per Jesus' instructions just after he was resurrected. In Galatians (and corroborated in Corinthians) Paul mentions a period of 14 years after his conversion when he finally goes to Jerusalem to meet with the apostles. If Paul converted before 36, then this refers to a time prior to 50 CE., when at least some apostles were still there. Peter and John are mentioned specifically, and James who was also mentioned had not been considered an apostle. Acts also does not mention any additional apostles (except Peter and James) still alive or around at this time. If there had been an instruction to all the apostles to stay in Jerusalem, for the purpose of forming an apostolic group to study the scriptures and devote themselves to prayer, then it may have already accomplished its purpose and broken up before 50 CE.
We hear of the "The Twelve" in Acts 6, still in Jerusalem, when they send out Stephen and Philip, for example. (Still prior to Paul's conversion.) By chapter 8 of Acts we hear about the group of "apostles in Jerusalem," and how both Peter and John had been sent to Samaria (and we know from Galatians that Peter had gone as far as Antioch). But by chapter 11 of Acts we only hear of some older men in Jerusalem and only one apostle there, Peter. By the time we reach 11:29 we only hear about the effect of the famine on "the brothers living in Judea." This matches about the same timing as Paul spoke of in Galatians and elsewhere when Paul brought collections back to the brothers in Jerusalem "keeping the poor in mind." Then in the next verses of Acts (Acts 12:1-3) we see that Herod has just put the Apostle James (brother of John) to death (not the same James of "James, Peter, and John" in Galatians) and goes after Peter.
After the destruction, we see John the apostle up around the isle of Patmos, but this could have been an exile from anywhere, not necessarily Jerusalem.
So, we really don't know how long the apostles stayed together in Jerusalem, or whether Herod broke that up even prior to the work of Paul and Barnabas, that brought them to Jerusalem (Acts 15). No Bible writer mentions Pella. I don't think there is anyone we know about who mentioned anything about a flight to Pella until 200 years later. There are plenty of letters and stories and other Christian writings between 70 and 300, but no evidence about Pella.
Still, we have the history (through Josephus) of the attack on Jerusalem, and the fact that the Romans started to attack in 66 and then decided to withdraw and not come back until they were ready to wage the war from start to finish in 70. Perhaps no Christians left in 66, although this seems like the time that would fit best. It's possible that most Christians had already left during Herod's persecution. It's possible that most Christians left just as the final approach was being made in 70 around the time of the Passover.
The most "ideal" story says that Christians recognized Jesus warning when Jerusalem was approached in 66, and that they then left and stayed away for about a 3-and-a-half year period until Jerusalem was destroyed with its Temple in 70. But we have no evidence from Josephus or anyone else about that.
It might also be wishful thinking to believe that no Christians were killed during the destruction of Jerusalem, as Epiphanius claimed 300 years later.
As to the idea that John was the only apostle alive after 70, there is an interesting passage in John that seems to refer to his age and the timing of the parousia.
(John 21:20-24) . . .Peter turned around and saw the disciple whom Jesus loved following, . . . 21 So when he caught sight of him, Peter said to Jesus: “Lord, what about this man?” 22 Jesus said to him: “If it is my will for him to remain until I come, of what concern is that to you? You continue following me.” 23 So the saying went out among the brothers that this disciple would not die. However, Jesus did not say to him that he would not die, but he said: “If it is my will for him to remain until I come, of what concern is that to you?” 24 This is the disciple who gives this witness about these things and who wrote these things, and we know that his witness is true.
I like your questions and there is much more to say about them, but I'll stop for now.

Naturally, if it's something that might be historical but isn't found in the Bible itself, the first place you'd look is in the writings of contemporaries of first century Christians like
Josephus, Pliny the Elder, Pliny the Younger You might also try persons who referenced famous persons from the first century in works that referenced say Philo or Gamaliel, etc., even though they died before the Temple was destroyed. The same would go for biographies of the Romans or their military exploits that might reference:
Herod Agrippa Cestius Gallus Gessius Florus Nero, Galba, Otho, Vitellius, and Vespasian. (the 5 Roman Emperors between 66 and 70) Then you might try to find references in or about the writings of any of the following historians who might have mentioned something by chance up to within a 100 years of the Jerusalem event:
Marcus Cluvius Rufus, (fl. 41–69), Roman history Quintus Curtius Rufus (c. 60–70), Greek history Flavius Josephus (37–100), Jewish history Dio Chrysostom (c. 40 – c. 115 AD), history of the Getae Thallus (early 2nd century AD), Roman history Gaius Cornelius Tacitus (c. 56–120), early Roman Empire Plutarch (c. 46–120), Parallel Lives of important Greeks and Romans Criton of Heraclea (fl. 100), history of the Getae and the Dacian Wars Suetonius (c. 69 – after 122), Roman emperors up to the Flavian dynasty Appian (c. 95 – c. 165), Roman history Arrian (c. 92–175), Greek history Granius Licinianus (2nd century), Roman history Criton of Pieria (2nd century), Greek history Lucius Ampelius (c. 2nd century AD), Roman history Dio Cassius (c. 160 – after 229), Roman history Marius Maximus (c. 160 – c. 230), biographer of Roman emperors Diogenes Laërtius (fl. c. 230), history of Greek philosophers Sextus Julius Africanus (c. 160 – c. 240), early Christian The many Jewish and Jewish Christian writings to check during this period could include those with dates on them that fit the period, as seen on earlyjewishwritings.com and earlychristianwritings.com. Note that the dates are usually considered to be "scholarly" dates, not the dates that Christians assign to them when it comes to the actual NT writings. That requires a whole new discussion, but for now, those writings to check would include the following:
(Excuse the formatting issues below. I just copied them from a portion of the list at earlychristianwritings.com. It's a couple years worth of reading. I haven't completed more than a few of them, but have found nothing yet about the flight to Pella/Perea/Decapolis in any of them. The list of writings becomes much longer if you include Christian-related writings all the way up through the 300's when Eusebius and Epiphanius wrote.)
50-95 Signs Gospel 50-95 Book of Hebrews 50-120 Didache 50-140 Gospel of Thomas 50-140 Oxyrhynchus 1224 Gospel 50-150 Apocalypse of Adam 50-150 Eugnostos the Blessed 50-200 Sophia of Jesus Christ 65-80 Gospel of Mark 70-100 Epistle of James 70-120 Egerton Gospel 70-160 Gospel of Peter 70-160 Secret Mark 70-200 Fayyum Fragment 70-200 Testaments of the Twelve Patriarchs 73-200 Mara Bar Serapion 80-100 2 Thessalonians 80-100 Ephesians 80-100 Gospel of Matthew 80-110 1 Peter 80-120 Epistle of Barnabas 80-130 Gospel of Luke 80-130 Acts of the Apostles 80-140 1 Clement 80-150 Gospel of the Egyptians 80-150 Gospel of the Hebrews 80-250 Christian Sibyllines 90-95 Revelation 90-120 Gospel of John 90-120 1 John 90-120 2 John 90-120 3 John 90-120 Epistle of Jude 93 Flavius Josephus 100-150 1 Timothy 100-150 2 Timothy 100-150 Titus 100-150 Apocalypse of Peter 100-150 Secret Book of James 100-150 Preaching of Peter 100-160 Gospel of the Ebionites 100-160 Gospel of the Nazoreans 100-160 Shepherd of Hermas 100-160 2 Peter 100-200 Odes of Solomon 100-200 Gospel of Eve 100-230 Thunder, Perfect Mind 101-220 Book of Elchasai 105-115 Ignatius of Antioch 110-140 Polycarp to the Philippians 110-140 Papias 110-160 Oxyrhynchus 840 Gospel 110-160 Traditions of Matthias 111-112 Pliny the Younger 115 Suetonius 115 Tacitus 120-130 Quadratus of Athens 120-130 Apology of Aristides 120-140 Basilides 120-140 Naassene Fragment 120-160 Valentinus 120-180 Apocryphon of John 120-180 Gospel of Mary 120-180 Dialogue of the Savior 120-180 Gospel of the Savior 120-180 2nd Apocalypse of James 120-180 Trimorphic Protennoia 120-180 Gospel of Perfection 120-200 Genna Marias 130-140 Marcion 130-150 Aristo of Pella 130-160 Epiphanes On Righteousness 130-160 Ophite Diagrams 130-160 2 Clement 130-170 Gospel of Judas 130-200 Epistle of Mathetes to Diognetus 140-150 Epistula Apostolorum   140-160 Ptolemy 140-160 Isidore 140-170 Fronto 140-170 Infancy Gospel of James 140-170 Infancy Gospel of Thomas 140-180 Gospel of Truth 150-160 Martyrdom of Polycarp 150-160 Justin Martyr 150-180 Excerpts of Theodotus 150-180 Heracleon 150-200 Ascension of Isaiah 150-200 Interpretation of Knowledge 150-200 Testimony of Truth 150-200 Acts of Peter 150-200 Acts of John 150-200 Acts of Paul 150-200 Acts of Andrew 150-225 Acts of Peter and the Twelve 150-225 Book of Thomas the Contender 150-250 Paraphrase of Shem 150-250 Fifth and Sixth Books of Esra 150-300 Authoritative Teaching 150-300 Coptic Apocalypse of Paul 150-300 Prayer of the Apostle Paul 150-300 Discourse on the Eighth and Ninth 150-300 Melchizedek 150-350 Preaching of Paul 150-350 Epistle to the Laodiceans 150-350 Questions of Mary 150-350 Allogenes, the Stranger 150-350 Hypsiphrone 150-350 Valentinian Exposition 150-350 Act of Peter 150-360 Concept of Our Great Power 150-400 Acts of Pilate 150-400 Anti-Marcionite Prologues 150-400 Dialogue Between John and Jesus 160-170 Tatian's Address to the Greeks 160-180 Claudius Apollinaris 160-180 Apelles 160-180 Julius Cassianus 160-250 Octavius of Minucius Felix 161-180 Acts of Carpus 165-175 Melito of Sardis 165-175 Hegesippus 165-175 Dionysius of Corinth 165-175 Lucian of Samosata 167 Marcus Aurelius 170-175 Diatessaron 170-200 Dura-Europos Gospel Harmony 170-200 Muratorian Canon 170-200 Treatise on the Resurrection 170-220 Letter of Peter to Philip 170-230 Thought of Norea 175-180 Athenagoras of Athens 175-185 Irenaeus of Lyons 175-185 Rhodon 175-185 Theophilus of Caesarea 175-190 Galen 178 Celsus 178 Letter from Vienna and Lyons 180 Passion of the Scillitan Martyrs 180-185 Theophilus of Antioch 180-185 Acts of Apollonius

Using the quotes extracted from Eusebius and Epaphanius in a Wiki article https://en.wikipedia.org/wiki/Flight_to_Pella
It might be interesting to note that the impetus to leave Jerusalem and go to Pella was not specifically credited to Jesus' prophecy in Matthew/Mark/Luke, but to an angel, or a specific oracle/revelation/prophecy given just before the war. This would put it on par with the prophecy of Agabus (Acts 11:27, 28) . . .In those days prophets came down from Jerusalem to Antioch. 28 One of them named Agʹa·bus stood up and foretold through the spirit that a great famine was about to come on the entire inhabited earth, which, in fact, did take place in the time of Claudius.
This is a curious report then by Eusebius, that he doesn't tie it to Matthew 24, or Luke 21, for example. (Epiphanius may have "corrected" this nearly 100 years after Eusebius, when he credited the flight to Jesus warning about the city being surrounded.)
The idea that the command only went to those in the city who were worthy, might also imply that there were reports that some [less worthy] Christians had died in Jerusalem's destruction.   Epaphanius had referenced Jesus' prophecy in his book Panarion, but in "Weights and Measures" he pretty much agreed with Eusebius:  

Unfortunately, this has been going on even from the time that the scriptures were written, or at least from very shortly after the NT was completed. This means that even the very idea that there had been an escape to Pella might just be from persons with their own agenda.
The best evidence that comes down to us is from Eusebius of Caesarea and Epiphanius of Salamis. Eusebius wrote his "Ecclesiastical History" (Church History) between about 300 and 325. Epiphanius would have written "Panarion" around 375. We don't know what, if anything, was written on this topic between 70 CE and 300 CE.
So this might be a little like someone just now trying to make a story about what direction we believe small bands of native Americans (Indians) ran to in 1775 in upstate NY when Ethan Allen and Benedict Arnold were capturing the guns at the British Fort Ticonderoga (which would just precede a siege of Boston and the building of a fortification of stakes around parts of that city).
The "American Indians" were not a big part of that story, so there is very little written down about what they did. If someone came up with a new story about it 200 to 250 years later, we might not put much trust in it. But, then again, we might assume that there were some verbal or even written records that could be gathered up from various families in the area, and that there was some truth to such a story.
We have some evidence that the apostles generally stayed in Jerusalem, per Jesus' instructions just after he was resurrected. In Galatians (and corroborated in Corinthians) Paul mentions a period of 14 years after his conversion when he finally goes to Jerusalem to meet with the apostles. If Paul converted before 36, then this refers to a time prior to 50 CE., when at least some apostles were still there. Peter and John are mentioned specifically, and James who was also mentioned had not been considered an apostle. Acts also does not mention any additional apostles (except Peter and James) still alive or around at this time. If there had been an instruction to all the apostles to stay in Jerusalem, for the purpose of forming an apostolic group to study the scriptures and devote themselves to prayer, then it may have already accomplished its purpose and broken up before 50 CE.
We hear of the "The Twelve" in Acts 6, still in Jerusalem, when they send out Stephen and Philip, for example. (Still prior to Paul's conversion.) By chapter 8 of Acts we hear about the group of "apostles in Jerusalem," and how both Peter and John had been sent to Samaria (and we know from Galatians that Peter had gone as far as Antioch). But by chapter 11 of Acts we only hear of some older men in Jerusalem and only one apostle there, Peter. By the time we reach 11:29 we only hear about the effect of the famine on "the brothers living in Judea." This matches about the same timing as Paul spoke of in Galatians and elsewhere when Paul brought collections back to the brothers in Jerusalem "keeping the poor in mind." Then in the next verses of Acts (Acts 12:1-3) we see that Herod has just put the Apostle James (brother of John) to death (not the same James of "James, Peter, and John" in Galatians) and goes after Peter.
After the destruction, we see John the apostle up around the isle of Patmos, but this could have been an exile from anywhere, not necessarily Jerusalem.
So, we really don't know how long the apostles stayed together in Jerusalem, or whether Herod broke that up even prior to the work of Paul and Barnabas, that brought them to Jerusalem (Acts 15). No Bible writer mentions Pella. I don't think there is anyone we know about who mentioned anything about a flight to Pella until 200 years later. There are plenty of letters and stories and other Christian writings between 70 and 300, but no evidence about Pella.
Still, we have the history (through Josephus) of the attack on Jerusalem, and the fact that the Romans started to attack in 66 and then decided to withdraw and not come back until they were ready to wage the war from start to finish in 70. Perhaps no Christians left in 66, although this seems like the time that would fit best. It's possible that most Christians had already left during Herod's persecution. It's possible that most Christians left just as the final approach was being made in 70 around the time of the Passover.
The most "ideal" story says that Christians recognized Jesus warning when Jerusalem was approached in 66, and that they then left and stayed away for about a 3-and-a-half year period until Jerusalem was destroyed with its Temple in 70. But we have no evidence from Josephus or anyone else about that.
It might also be wishful thinking to believe that no Christians were killed during the destruction of Jerusalem, as Epiphanius claimed 300 years later.
As to the idea that John was the only apostle alive after 70, there is an interesting passage in John that seems to refer to his age and the timing of the parousia.
(John 21:20-24) . . .Peter turned around and saw the disciple whom Jesus loved following, . . . 21 So when he caught sight of him, Peter said to Jesus: “Lord, what about this man?” 22 Jesus said to him: “If it is my will for him to remain until I come, of what concern is that to you? You continue following me.” 23 So the saying went out among the brothers that this disciple would not die. However, Jesus did not say to him that he would not die, but he said: “If it is my will for him to remain until I come, of what concern is that to you?” 24 This is the disciple who gives this witness about these things and who wrote these things, and we know that his witness is true.
I like your questions and there is much more to say about them, but I'll stop for now.

(Matthew 24:15, 16) 15 “Therefore, when you catch sight of the disgusting thing that causes desolation, as spoken about by Daniel the prophet, standing in a holy place (let the reader use discernment), 16 then let those in Ju·deʹa begin fleeing to the mountains.
(Mark 13:14) 14 “However, when you catch sight of the disgusting thing that causes desolation standing where it should not be (let the reader use discernment), then let those in Ju·deʹa begin fleeing to the mountains.
(Luke 21:20-28) 20 “However, when you see Jerusalem surrounded by encamped armies, then know that the desolating of her has drawn near. 21 Then let those in Ju·deʹa begin fleeing to the mountains, let those in the midst of her leave, and let those in the countryside not enter into her, 22 because these are days for meting out justice in order that all the things written may be fulfilled. 23 Woe to the pregnant women and those nursing a baby in those days! For there will be great distress on the land and wrath against this people. 24 And they will fall by the edge of the sword and be led captive into all the nations; and Jerusalem will be trampled on by the nations until the appointed times of the nations are fulfilled. 25 “Also, there will be signs in the sun and moon and stars, and on the earth anguish of nations not knowing the way out because of the roaring of the sea and its agitation. 26 People will become faint out of fear and expectation of the things coming upon the inhabited earth, for the powers of the heavens will be shaken. 27 And then they will see the Son of man coming in a cloud with power and great glory. 28 But as these things start to occur, stand up straight and lift up your heads, because your deliverance is getting near.”
The words of Mark and Matthew implied that the Romans would have come right up to the Temple to defile it, and that this was the time to leave as quickly as possible. We know from Josephus that Jews read Daniel's "abomination of desolation" to be based on Antiochus IV, who: according to common knowledge had done as follows, 200 years earlier:
In 168 BC, the Greek king Antiochus IV Epiphanes invaded Jerusalem and captured the city. He marched into the Jewish temple, erected a statue of the Greek god Zeus, and sacrificed a pig on the altar of incense. This provoked a revolt in Judea as the Jews fought to remove Antiochus’ sacrilege from the temple. https://www.ligonier.org/learn/devotionals/abomination-temple/
But Titus did not do anything akin to this in 66. Although in 70, he did. You can see almost direct evidence of this today by looking at the Arch of Titus.
In that year, the Roman general Titus invaded Jerusalem to crush a Jewish revolt, entered the temple, had the building destroyed, and carried off the lampstand and other temple artifacts to Rome.
So, it seems likely that it was not specifically anything in 66 (in Jesus' prophecy) that would have triggered a fleeing to Pella, nor does anyone who believed in a Pella flight actually time it to 66. Cestius Gallus did plunder the Temple [funds] and it resulted in a counter-attack by the Jews that was mostly successful. So this was an excellent time to leave, and both Jews and Romans got out of the city at that time. Wikipedia says:
The Roman governor, Gessius Florus, responded by plundering the Second Temple, claiming the money was for the Emperor, and the next day launching a raid on the city, arresting numerous senior Jewish figures. This prompted a wider, large-scale rebellion and the Roman military garrison of Judaea was quickly overrun by the rebels, while the pro-Roman king Herod Agrippa II, together with Roman officials, fled Jerusalem. As it became clear the rebellion was getting out of control, Cestius Gallus, the legate of Syria, brought in the Syrian army, based on Legion XII Fulminata and reinforced by auxiliary troops, to restore order and quell the revolt. Despite initial advances and the conquest of Jaffa, the Syrian Legion was ambushed and defeated by Jewish rebels at the Battle of Beth Horon with 6,000 Romans massacred and the Legion's aquila lost.
But at that point, Christians would have many months of opportunities to leave the city between 66 and the actual surrounding of Jerusalem in 70. 66 fits some of what Jesus said, but it was not a case so desperate that one would be unable to even grab a coat from inside your house. That was more like the situation just before Passover in 70. This is probably why Eusebius, who had read both Josephus and knew the Bible very well, believed the fleeing to Pella to be based on an angelic revelation.

Using the quotes extracted from Eusebius and Epaphanius in a Wiki article https://en.wikipedia.org/wiki/Flight_to_Pella
It might be interesting to note that the impetus to leave Jerusalem and go to Pella was not specifically credited to Jesus' prophecy in Matthew/Mark/Luke, but to an angel, or a specific oracle/revelation/prophecy given just before the war. This would put it on par with the prophecy of Agabus (Acts 11:27, 28) . . .In those days prophets came down from Jerusalem to Antioch. 28 One of them named Agʹa·bus stood up and foretold through the spirit that a great famine was about to come on the entire inhabited earth, which, in fact, did take place in the time of Claudius.
This is a curious report then by Eusebius, that he doesn't tie it to Matthew 24, or Luke 21, for example. (Epiphanius may have "corrected" this nearly 100 years after Eusebius, when he credited the flight to Jesus warning about the city being surrounded.)
The idea that the command only went to those in the city who were worthy, might also imply that there were reports that some [less worthy] Christians had died in Jerusalem's destruction.   Epaphanius had referenced Jesus' prophecy in his book Panarion, but in "Weights and Measures" he pretty much agreed with Eusebius:  

I just went to that topic for the first time, and it looks like an admin locked it. Too bad, it might have been entertaining.
I can understand how JTR feels about this topic, as it has been played out so many times with various interpretations. I have a great interest in what it would have meant to the Bible writers who included the idea, what it would have meant to the first hearers and readers. All scripture is beneficial for teaching, and things that befell earlier generations of God's people were often written as warning examples for the rest of us in later generations. This does not mean we must find a later "fulfillment," specifically, but we should be able to discern warnings, examples, and lessons for our day, because we all go through similar trials. There is nothing new under the sun.
The Bible clearly speaks of the last days starting in the first century. Paul warned Timothy about why persons in Timothy's congregation(s) were acting the way they were (men will be lovers of themselves, without natural affection, not open to any agreement, etc.).  Paul explained to Timothy that this was to be expected back there in the first century because they were living in the "last days" when "critical times hard to deal with" were to be expected. Peter and Jude also explain that people in the first century would be ridiculing the "delay" of the parousia. And the reason this was expected was that Jesus had predicted that such attitudes would be prevalent in the last days. The book of Hebrews starts out identifying the time the Christians were living in as "the last days." (The exact same expression "last days" found in Timothy.) Peter reads Joel's prophecy in Acts 2 to explain that what happened at Pentecost was a sign of the last days. And John in his letters identifies those times as the last days. Even the book of Revelation speaks of how Daniel was a sealed up book, but it is being revealed through John because they had now reached the last days when it was time for those prophecies to be unsealed.
So there was something very "big" about the fulfillment of the prophecy of the destruction of Jerusalem in the first century. This was the generation that saw the judgment which meant the transferring of the Messianic kingdom over to Christians who needed not to have ever been involved in Judaism. This was the birth of a new nation. It was every bit as important to world history as when Jehovah chose Abraham for a promised nation to come from his loins.
That generation likely saw the major fulfillment of all those prophecies of Daniel, Ezekiel, Zechariah and Revelation and Matthew 23 and 24. The fulfillment left for us is the overall "lesson" that Israel went through, but I don't think that we should be looking for any additional specific fulfillments. If there were additional fulfillments to look for today, prior to Armageddon, then the day could not come as a thief. (Of course, there MIGHT be specific fulfillments predicted in Scripture, but if so, they should not be of the kind that we could discern in advance or else this would conflict with the Great Day coming as a thief.)
Of course, some will argue that even if specific prophecies are not predictable by the world (in darkness) that Paul said that the day should not overtake us (children of light) as a thief would. But that doesn't refer to prophecy, it just means that we are not going to be overwhelmed with fear, because we expect it at any time, and are prepared in the sense that we continue to watch the kind of person we ought to be at all times. But it still comes as a thief, even while persons are commenting on how things are peaceful and secure. Jesus and Peter say the same thing when they spoke of how life would be going on just as it was in Noah's day and Lot's day, as things had always gone on "from creation's beginning" until now.
Also, I think if we look closely at Daniel, Revelation, etc., that it becomes even clearer that we have no need to try to identify any current players as King of the North or South.

Using the quotes extracted from Eusebius and Epaphanius in a Wiki article https://en.wikipedia.org/wiki/Flight_to_Pella
It might be interesting to note that the impetus to leave Jerusalem and go to Pella was not specifically credited to Jesus' prophecy in Matthew/Mark/Luke, but to an angel, or a specific oracle/revelation/prophecy given just before the war. This would put it on par with the prophecy of Agabus (Acts 11:27, 28) . . .In those days prophets came down from Jerusalem to Antioch. 28 One of them named Agʹa·bus stood up and foretold through the spirit that a great famine was about to come on the entire inhabited earth, which, in fact, did take place in the time of Claudius.
This is a curious report then by Eusebius, that he doesn't tie it to Matthew 24, or Luke 21, for example. (Epiphanius may have "corrected" this nearly 100 years after Eusebius, when he credited the flight to Jesus warning about the city being surrounded.)
The idea that the command only went to those in the city who were worthy, might also imply that there were reports that some [less worthy] Christians had died in Jerusalem's destruction.   Epaphanius had referenced Jesus' prophecy in his book Panarion, but in "Weights and Measures" he pretty much agreed with Eusebius:  

Unfortunately, this has been going on even from the time that the scriptures were written, or at least from very shortly after the NT was completed. This means that even the very idea that there had been an escape to Pella might just be from persons with their own agenda.
The best evidence that comes down to us is from Eusebius of Caesarea and Epiphanius of Salamis. Eusebius wrote his "Ecclesiastical History" (Church History) between about 300 and 325. Epiphanius would have written "Panarion" around 375. We don't know what, if anything, was written on this topic between 70 CE and 300 CE.
So this might be a little like someone just now trying to make a story about what direction we believe small bands of native Americans (Indians) ran to in 1775 in upstate NY when Ethan Allen and Benedict Arnold were capturing the guns at the British Fort Ticonderoga (which would just precede a siege of Boston and the building of a fortification of stakes around parts of that city).
The "American Indians" were not a big part of that story, so there is very little written down about what they did. If someone came up with a new story about it 200 to 250 years later, we might not put much trust in it. But, then again, we might assume that there were some verbal or even written records that could be gathered up from various families in the area, and that there was some truth to such a story.
We have some evidence that the apostles generally stayed in Jerusalem, per Jesus' instructions just after he was resurrected. In Galatians (and corroborated in Corinthians) Paul mentions a period of 14 years after his conversion when he finally goes to Jerusalem to meet with the apostles. If Paul converted before 36, then this refers to a time prior to 50 CE., when at least some apostles were still there. Peter and John are mentioned specifically, and James who was also mentioned had not been considered an apostle. Acts also does not mention any additional apostles (except Peter and James) still alive or around at this time. If there had been an instruction to all the apostles to stay in Jerusalem, for the purpose of forming an apostolic group to study the scriptures and devote themselves to prayer, then it may have already accomplished its purpose and broken up before 50 CE.
We hear of the "The Twelve" in Acts 6, still in Jerusalem, when they send out Stephen and Philip, for example. (Still prior to Paul's conversion.) By chapter 8 of Acts we hear about the group of "apostles in Jerusalem," and how both Peter and John had been sent to Samaria (and we know from Galatians that Peter had gone as far as Antioch). But by chapter 11 of Acts we only hear of some older men in Jerusalem and only one apostle there, Peter. By the time we reach 11:29 we only hear about the effect of the famine on "the brothers living in Judea." This matches about the same timing as Paul spoke of in Galatians and elsewhere when Paul brought collections back to the brothers in Jerusalem "keeping the poor in mind." Then in the next verses of Acts (Acts 12:1-3) we see that Herod has just put the Apostle James (brother of John) to death (not the same James of "James, Peter, and John" in Galatians) and goes after Peter.
After the destruction, we see John the apostle up around the isle of Patmos, but this could have been an exile from anywhere, not necessarily Jerusalem.
So, we really don't know how long the apostles stayed together in Jerusalem, or whether Herod broke that up even prior to the work of Paul and Barnabas, that brought them to Jerusalem (Acts 15). No Bible writer mentions Pella. I don't think there is anyone we know about who mentioned anything about a flight to Pella until 200 years later. There are plenty of letters and stories and other Christian writings between 70 and 300, but no evidence about Pella.
Still, we have the history (through Josephus) of the attack on Jerusalem, and the fact that the Romans started to attack in 66 and then decided to withdraw and not come back until they were ready to wage the war from start to finish in 70. Perhaps no Christians left in 66, although this seems like the time that would fit best. It's possible that most Christians had already left during Herod's persecution. It's possible that most Christians left just as the final approach was being made in 70 around the time of the Passover.
The most "ideal" story says that Christians recognized Jesus warning when Jerusalem was approached in 66, and that they then left and stayed away for about a 3-and-a-half year period until Jerusalem was destroyed with its Temple in 70. But we have no evidence from Josephus or anyone else about that.
It might also be wishful thinking to believe that no Christians were killed during the destruction of Jerusalem, as Epiphanius claimed 300 years later.
As to the idea that John was the only apostle alive after 70, there is an interesting passage in John that seems to refer to his age and the timing of the parousia.
(John 21:20-24) . . .Peter turned around and saw the disciple whom Jesus loved following, . . . 21 So when he caught sight of him, Peter said to Jesus: “Lord, what about this man?” 22 Jesus said to him: “If it is my will for him to remain until I come, of what concern is that to you? You continue following me.” 23 So the saying went out among the brothers that this disciple would not die. However, Jesus did not say to him that he would not die, but he said: “If it is my will for him to remain until I come, of what concern is that to you?” 24 This is the disciple who gives this witness about these things and who wrote these things, and we know that his witness is true.
I like your questions and there is much more to say about them, but I'll stop for now.

Unfortunately, this has been going on even from the time that the scriptures were written, or at least from very shortly after the NT was completed. This means that even the very idea that there had been an escape to Pella might just be from persons with their own agenda.
The best evidence that comes down to us is from Eusebius of Caesarea and Epiphanius of Salamis. Eusebius wrote his "Ecclesiastical History" (Church History) between about 300 and 325. Epiphanius would have written "Panarion" around 375. We don't know what, if anything, was written on this topic between 70 CE and 300 CE.
So this might be a little like someone just now trying to make a story about what direction we believe small bands of native Americans (Indians) ran to in 1775 in upstate NY when Ethan Allen and Benedict Arnold were capturing the guns at the British Fort Ticonderoga (which would just precede a siege of Boston and the building of a fortification of stakes around parts of that city).
The "American Indians" were not a big part of that story, so there is very little written down about what they did. If someone came up with a new story about it 200 to 250 years later, we might not put much trust in it. But, then again, we might assume that there were some verbal or even written records that could be gathered up from various families in the area, and that there was some truth to such a story.
We have some evidence that the apostles generally stayed in Jerusalem, per Jesus' instructions just after he was resurrected. In Galatians (and corroborated in Corinthians) Paul mentions a period of 14 years after his conversion when he finally goes to Jerusalem to meet with the apostles. If Paul converted before 36, then this refers to a time prior to 50 CE., when at least some apostles were still there. Peter and John are mentioned specifically, and James who was also mentioned had not been considered an apostle. Acts also does not mention any additional apostles (except Peter and James) still alive or around at this time. If there had been an instruction to all the apostles to stay in Jerusalem, for the purpose of forming an apostolic group to study the scriptures and devote themselves to prayer, then it may have already accomplished its purpose and broken up before 50 CE.
We hear of the "The Twelve" in Acts 6, still in Jerusalem, when they send out Stephen and Philip, for example. (Still prior to Paul's conversion.) By chapter 8 of Acts we hear about the group of "apostles in Jerusalem," and how both Peter and John had been sent to Samaria (and we know from Galatians that Peter had gone as far as Antioch). But by chapter 11 of Acts we only hear of some older men in Jerusalem and only one apostle there, Peter. By the time we reach 11:29 we only hear about the effect of the famine on "the brothers living in Judea." This matches about the same timing as Paul spoke of in Galatians and elsewhere when Paul brought collections back to the brothers in Jerusalem "keeping the poor in mind." Then in the next verses of Acts (Acts 12:1-3) we see that Herod has just put the Apostle James (brother of John) to death (not the same James of "James, Peter, and John" in Galatians) and goes after Peter.
After the destruction, we see John the apostle up around the isle of Patmos, but this could have been an exile from anywhere, not necessarily Jerusalem.
So, we really don't know how long the apostles stayed together in Jerusalem, or whether Herod broke that up even prior to the work of Paul and Barnabas, that brought them to Jerusalem (Acts 15). No Bible writer mentions Pella. I don't think there is anyone we know about who mentioned anything about a flight to Pella until 200 years later. There are plenty of letters and stories and other Christian writings between 70 and 300, but no evidence about Pella.
Still, we have the history (through Josephus) of the attack on Jerusalem, and the fact that the Romans started to attack in 66 and then decided to withdraw and not come back until they were ready to wage the war from start to finish in 70. Perhaps no Christians left in 66, although this seems like the time that would fit best. It's possible that most Christians had already left during Herod's persecution. It's possible that most Christians left just as the final approach was being made in 70 around the time of the Passover.
The most "ideal" story says that Christians recognized Jesus warning when Jerusalem was approached in 66, and that they then left and stayed away for about a 3-and-a-half year period until Jerusalem was destroyed with its Temple in 70. But we have no evidence from Josephus or anyone else about that.
It might also be wishful thinking to believe that no Christians were killed during the destruction of Jerusalem, as Epiphanius claimed 300 years later.
As to the idea that John was the only apostle alive after 70, there is an interesting passage in John that seems to refer to his age and the timing of the parousia.
(John 21:20-24) . . .Peter turned around and saw the disciple whom Jesus loved following, . . . 21 So when he caught sight of him, Peter said to Jesus: “Lord, what about this man?” 22 Jesus said to him: “If it is my will for him to remain until I come, of what concern is that to you? You continue following me.” 23 So the saying went out among the brothers that this disciple would not die. However, Jesus did not say to him that he would not die, but he said: “If it is my will for him to remain until I come, of what concern is that to you?” 24 This is the disciple who gives this witness about these things and who wrote these things, and we know that his witness is true.
I like your questions and there is much more to say about them, but I'll stop for now.

It seems unlikely to me too. But although I don't expect any brothers to be underhanded, I have already seen evidence of underhandedness in this area. And it is always in the direction of making sure the WTS doesn't look as bad as it would have looked, had the spirit of the law been followed. This probably helped the WTS reputation during the years when nearly everyone else (colleges, NDA's, Vatican) was doing the same. Today, however, there are still cases even from relatively recently where the WTS goal appears to have been as follows:
Don't allow a case to go public unless absolutely legally necessary, and if it seems like invoking clergy privilege can help avoid allowing a case to go public, then argue that, too. Before revealing something to authorities yourself, always find out if you legally are required to, first.
The current Watchtower view and process is excellent because it recognizes that in the current state of affairs, this would be seen as a very shortsighted goal. "The end would be worse than the beginning" if any concealing from public scrutiny might be seen as a cover-up.
To my mind, and I could be wrong, Australian cases already showed signs of being managed in an "underhanded" way, when it appeared that this data matched the claims and the sense I was getting from cases that were referenced and discussed, and the admissions of the elders who were questioned. I would agree with CC that it's not likely all of these cases represented actual crimes, but even accounting for that, not one of the actual crimes was ever reported to proper authorities.
But I'm also trying to understand some additional informatin I was made aware of through a personal contact, now at Warwick. It was that the former Branch Overseer of Australia, who would have been questioned by the ARC, had fled the country/continent. I was even told what country he went to. Later I heard online reports (likely from ex-JWs) matching this "gossip" from an elder at Warwick, plus an additional online report (no additional confirmation) of a second Branch "official" who also fled the country.

Thanks for drawing my attention to that. I must remember to use the word evidence rather than proof. "Evidence" is a lot more forgiving 😃

This could easily be true. It's also possible that a JW in good standing at the Australian Branch leaked it. It's also possible that it was cleaned up to make it look like many fewer Elders and MS were involved. Some things seem very unlikely to us because we all have preconceived ideas about what such data would look like.
I responded when I saw this because the word "proof" and "proves" were being used. This is evidence, not proof. I think that JTR recognized this when he said we needed "hard data." None of us collected the evidence. None of us produced the spreadsheet. I'm confident that this is good evidence, but that doesn't make it proof.
For example, we don't know but that perhaps all Elders and MS were supposed to have been mostly removed from that sheet before it was handed over. This seems unlikely, but things like this have happened. Statistics are often manipulated. On a much more minor topic I gave my own experience where I was "in the room where it happened" as one particular statistic was changed so that the WTS appeared to have a better report than it actually got when the statistic was announced. 
Also, TTH, the abusers reported were not 100% clergy in all the other reports given over to the ARC. In fact, some of the commentary about other entities run by churches, but not by clergy were part of the investigation.
Outside of the ARC, I have only followed a few cases haphazardly, but I saw several reports where I could tell they were not just including "clergy." The Southern Baptist cases, for example, included many church volunteers along with clergy. Wikipedia says:
On February 10, 2019, the Houston Chronicle and San Antonio Express-News published an investigative report into widespread sexual abuse within member churches of the Southern Baptist Convention. The report found since 1998, roughly 380 clergy, lay leaders and volunteers had faced allegations of sexual misconduct, leaving behind over 700 victims.[1]

This could easily be true. It's also possible that a JW in good standing at the Australian Branch leaked it. It's also possible that it was cleaned up to make it look like many fewer Elders and MS were involved. Some things seem very unlikely to us because we all have preconceived ideas about what such data would look like.
I responded when I saw this because the word "proof" and "proves" were being used. This is evidence, not proof. I think that JTR recognized this when he said we needed "hard data." None of us collected the evidence. None of us produced the spreadsheet. I'm confident that this is good evidence, but that doesn't make it proof.
For example, we don't know but that perhaps all Elders and MS were supposed to have been mostly removed from that sheet before it was handed over. This seems unlikely, but things like this have happened. Statistics are often manipulated. On a much more minor topic I gave my own experience where I was "in the room where it happened" as one particular statistic was changed so that the WTS appeared to have a better report than it actually got when the statistic was announced. 
Also, TTH, the abusers reported were not 100% clergy in all the other reports given over to the ARC. In fact, some of the commentary about other entities run by churches, but not by clergy were part of the investigation.
Outside of the ARC, I have only followed a few cases haphazardly, but I saw several reports where I could tell they were not just including "clergy." The Southern Baptist cases, for example, included many church volunteers along with clergy. Wikipedia says:
On February 10, 2019, the Houston Chronicle and San Antonio Express-News published an investigative report into widespread sexual abuse within member churches of the Southern Baptist Convention. The report found since 1998, roughly 380 clergy, lay leaders and volunteers had faced allegations of sexual misconduct, leaving behind over 700 victims.[1]