I haven't read the thread, too long! Just jumped in, sorry if I've repeated some of your points.
The most reasonable guess at Jesus's message imo is that Jesus is proclaiming the imminent coming of God's Kingdom to his fellow Jews, the chosen people who have turned their back on their God. When the Kingdom of God comes, they will be Judged, and thus need to repent.
That is pretty much my take on it, except I’m not sure about the repentance part. Each year for the Jews, then and now,there is a day of judgment, Rosh Hashanah, and a day of atonement, Yom Kippur, that I think would have covered it. As I understand it the messiah was to be a redeemer who freed the people from the political oppression of Caesar. Hence the title “king of the Jews”.
You could be right about the 'repent' part, it might well be a post-Jesus reinterpretation, eventually Jesus being seen as the suffering saviour/sacrificial scapegoat upon whom all our sins are placed giving us absolution. It's impossible to know how much post-resurrection reinterpretation of Jesus's life and teaching was inserted into the writings.
But Mark, one the foundational texts, doesn't seem to me to be there yet. His story is that this literal earth changing event has happened, and you'd better listen up. The old order is to be upturned, it didn't work (look around, our god has forsaken us), a new covenant of sorts is happening. The author (probably a Roman gentile around 60 AD) opens not with where Jesus was born or any biography, but with this dramatic miraculous scene. The symbolic ushering in of the new order, as John the messenger, the (heretical?) baptiser who washes away the sins of repenters, introduces the Messiah, the vehicle of the change, and his first words from Jesus are that the Kingdom of God is imminent, repent. (He ends with the empty tomb, a cliff-hanger rather than the story of the risen Jesus, and what that means). Why John the Baptist? If we assume Jesus was a follower of John, or had some association with him, then Jesus probably adhered to John's (this wacky wildman in the desert who'd developed a following) apparently idiosyncratic repentence theory. Maybe...
And perhaps this is the threat which Jesus poses to the Jewish authorities, and peace. He's implicitly saying the Sanhedrin has let the people down (hence Yahweh has abandoned them), the old ways either haven't worked or haven't been properly followed. Change is coming, but it's a different kind of Messiah, a spiritual one, heralding in a new spiritual world. And if Jesus really did enter Jerusalem at Passover with a lot of hoo-hah, the big time of the year where the town was packed and minds were focussed on religion, a potentially volatile day - that could be seen as provocative to many Jews, especially the authorities. The story the Jewish authorities might have given the Romans is hard to know, but for them both Jesus was potential trouble at a volatile time.
Who knows, but it's a story which makes sense to me, anyway. As you say, reading about the times really helps get a feel for how it might have been, but it's impossible to get into the head of the writer, or fully immersed in the world views of that time and place. I've read some Pagels and Ehrman who you mention, very readable authors, and there was a great podcast by some Canadian academic about those times, including all the competing ideas and groups floating around, but I've forgotten who did it now. And having studied the OT prophets in teeth grinding depth (they're all a blur now), I got a feel for how Jesus is part of a tradition when you come to study the NT, both a religious/cultural tradition, and a tradition of people who pop up at desperate times of crisis, when people are open to radical messages, looking for a saviour.
As I've said, the twist comes after his crucifixion (a very un-messianic fate), and the new post-Jesus story emerges of the resurrected messiah, eventually Jesus as part of the Godhead, and saviour of mankind. Returning any day now…It's this post-Jesus story which is spread to non-Jews. Along with Paul's doctrine of Justification by Faith, rather than by Works. Paul largely shaped the Christianity we've inherited, someone who never heard Jesus preach, and according to Acts was at logger-heads with the disciples who had.
That is how I see it as well. It is really surprising that people cannot see the central role of Paul, but then again, people often see what they want and not what is there to be seen.
It's hard to come to the texts with fresh eyes I think, they're so ingrained in our culture, the story already so familiar, usually having been learned as one coherent Jesus Story, with meanings supplied.
I talked a bit about the role of John earlier in this topic. After Paul he is the most important early figure in the development of Christianity. One significant fact is that Paul thought he would live to see the end of days. One might say that John did for the failure of Paul’s promise what Paul did for the failure of Jesus’ messianic promise (or what comes down to us as his promise). I mentioned earlier in the thread that Jesus would have been appalled to learn what had become of him.
. I re-read Mark a while ago, but I can't recall John so well. It's fascinating to compare their openings tho. John takes the same scene of the baptism, but by now John's Christology is literally out of this world compared to Mark.