The “gospels were dictated by first hand witness” idea is a massive problem because that’s not first hand account at all, that’s actually someone claiming that someone else told him “dude I swear I saw it happen in front of me as clear as I see you” (or worse, the guy who wrote it claims that he found this text written by someone else 50 years ago) and we somehow chose to believe both the guy who wrote it and the supposed guy who told him that. Having something dictated is second hand account, not first hand, because that’s just changing the pronoun of the person speaking. And there were extensive analysis of the text itself to try to figure out what kind of person would have phrased this or that in certain ways, whether it says “I saw that myself” or “my uncle who works at Nintendo told me he saw it himself”, and that analysis, done for the entirety of the Bible, has gone pretty far, including the gospels. As far as I know about it, the biggest point about that analysis is which gospel was written first and which ones copied from which ones or added their own thing, rahter than 4 different people recounting their memories of the same events.
I don’t know about the timeline of the temple; I’ve heard it brought up before, but I haven’t heard that it was considered conclusive evidence for dating the text, so I don’t know more than that and how it holds to the text analysis.