According to Hengel his name was attached immediately in the First century. So that is one piece of evidence.
If the conclusions of one scholar to that effect constitute evidence for that claim, then the contrary conclusions of other scholars constitute evidence against that claim.
And then the reports you mention...
The attribution seems to start with Papias, who lived roughly 60 A.D. to 130 A.D. according to Wikipedia. We have a citation of Papias, from much later, which has him simply saying that the disciple Matthew wrote a "logia" about Jesus. Scholars don't even agree about what this means, or whether it refers to the "Gospel of Matthew" as we have it.
combined with the unlikely attribution of such an obscure disciple to the book unless the evidence warranted it.
We don't know which disciples were "obscure" around 70 A.D. We don't know what was known, or assumed, or rumored about this particular disciple around that time, that might make his name attractive as the supposed author of this gospel. Maybe the fact that he was a tax collector implied that he was literate, unlike the typical fisherman, and so would make him a more plausible candidate.
No, he did not have a vision, he saw Christ in the flesh as shown by the men with him hearing Jesus' voice.
Hearing a voice is not the same as seeing somebody in the flesh. Acts 9 does not say anything explicitly (or implicitly, so far as I can see) about anybody -- either Saul or his companions -- seeing and recognizing Jesus. In any case, the reason for Paul's belief in the resurrected Jesus was apparently this experience, and not any consideration he'd given to reports from others.
Yes, the other independent source reports He appeared to 500 others, including Jesus' skeptic brother, who was martyred for his belief.
That is, Paul says that somebody said that he appeared to those others. Calling this an "independent source" is just a way of trying to elevate the credibility of what is obviously hearsay.
No, if it is a different author from several years before Paul even became a believer that is an independent source. Yes it would be source independent of Plutarch. There is no other definition of independent.
You are simply wrong about there being "no other definition of independent," as I noted when quoting the
Wikipedia article on "Multiple Independent Sources."
Actually many scholars date the creed to 36 AD. Yes, it does constitute strong historical evidence...
"Anonymous people are reported by a later source to have said something miraculous happened" is not "strong historical evidence" that the miraculous thing happened.
...combined with all the other evidence in the gospels.
Nor have you explained why the gospels constitute strong evidence.
Especially if some were sources that were independent...
Again,anonymous people who were reported by a later source as having said people saw the risen Christ are not "especially" powerful evidence for the resurrection.
...and some were formerly hostile like Paul and James.
Why are you counting James as "hostile" like Paul?
You have to be able to explain why these known skeptics would make a 180 degree change in such a short period. And in the case of James dying for that belief shortly afterwards.
According to Luke, Paul had an overwhelming experience on the road to Damascus. I'm not disputing that, or claiming he made it up. But such experiences are not something which others should be expected to trust.
As for the others, we have essentially no information about what made them continued followers of Christ, so their motivations -- what they believed, exactly, and why they believed it -- are pretty much entirely undiscoverable at this distance in time.