Jesus almost certainly claimed that he and his twelve disciples were going to rule over the new kingdom of God

Prove it.

We can be pretty sure because Judas then goes and betrays him, which we’re also very certain actually happened.

Prove it. Also explain why Paul doesn't seem to know about it.

Finally, Jesus probably actually was from Nazareth, because that town was basically like being from nowhere in those days. It would be strange to invent a story about a god and have them come from a podunk place, especially in those days where class mobility didn’t exist.

Not if you held to a doctrine that "the last shall be first and the first shall be last". A very popular theme in Jewish mysticism. Plus it would explain why there is no Jesus family around to claim the throne, except for James.

Furthermore, we can be pretty certain John the Baptist really did baptize Jesus. My understanding of why is a bit more limited but basically that action put John in a higher position of authority than Jesus which would have been a big issue. Scholars think Jesus was originally a disciple of John.

Criteria of embarrassment is the term you are looking for. That thing almost never used outside of biblical studies since it is a weak argument. It doesn't work here. John the Baptist was more well known than Jesus was at the time of Mark. By attaching Jesus to him it was just another form of name dropping. Additionally Mark has John still humbled in the role. So even less embarrassing for Christianity.

It’s not that there is one part of Mark where we can say, “Oh yes, Jesus really said that specific thing.” It’s Mark presents what is called a “low Christology”. That is, Jesus was a guy, who was especially wise and holy, and therefore rose up and become somehow more than a man and imbued with the supernatural. This is what his followers almost certainly believed about him during his life and shortly thereafter.

It is possible the James community thought that way but Paul certainly didn't. Also worth mentioning that Mark borrows the Latin fiction trope of the empty tomb meaning ascending to godhead. So even if Mark downplays Jesus while he was alive he makes him a god on his death.

Later gospels, especially John, present a “high Christology”: Jesus was with God on high and descended to earth and enlighten humanity, then went back to God.

Paul said nearly the same thing about a century earlier. What John added was the whole bit about the word. Paul does not talk about a normal person he talks about a celestial being who came to earth, did stuff, and unlike other humans (Paul didn't believe in an afterlife) came back to life in heavenly body form. Which meant he was the new Adam.

It’s been awhile since I’ve read the Gospels but I believe that there are some things Jesus says in Mark, like, “Don’t preach to the Greeks”, or “Avoid the Sumerians” that are right on the money, so to speak, about what a Jew form Judea at the time would have said. These statements are generally ignored by the modern Church because they contradict Christianity’s catholic/universal current state.

I am being honest and not snarky at all here but I have no idea what you are talking about. I did double check this morning and saw nothing like this in Mark. Could you quote the passage?