But you're missing the point. When one converts into a Jewish group, any Jewish group, one falls under the Law-- all of it. The idea that gentiles would be allowed into a Jewish group without being under the Covenant and the Law has no precedence. If they chose to remain separate, as the "God-fearers" did, that was fine and dandy, but they are not Jews inside a Jewish branch. The fact that they allowed gentiles in without full conversion should tell one something.
Then the gospels have less reliability than Paul's letters since they were written by and large later. And since all of them are quite subjective, there really isn't much room for any of it being reliable.
But which part is history and which is imagination and/or subjective fabrication, which is why I throw up my hands and just treat it all as allegory. Instead, I just deal with the basic teachings and see if any are applicable to my life. As far as history is concerned, whatever happened, happened; and speculation is just that-- speculation.
I do believe you're taking this out of context. Remember Paul's past, and imagine trying to put trust in a man that was persecuting your group earlier. Doncha think you're going to have some reservations? And doncha think you're gonna have some "issues" on top of that since Paul only met Jesus in a "vision", whereas the twelve knew Jesus personally? Why should you believe him would logically be a concern, especially when he keeps asking for donations, right?
So, what I believe Paul is saying is that he is authentic. BTW, if Peter and the others were hostile, or Paul was hostile to them, then why would they give Paul even the time of day? And why is it that the early church (still 1st century) honored his letters even more than the gospels at first? And one simply cannot say there were no Jews left or that there were none in any leadership positions.
No matter how one might look at it, as Joseph Campbell was fond of saying, "The myth became the reality".