>the New Testament comes as a unified set of 27 books. One of the first and primary rules of Bible interpretation is "Let the Bible explain itself." Since the Gospels are viewing the same individual from 4 vantage points, we do not separate them in interpretation.
I guess this just reveals a fundamental difference in the way we approach Scripture. The NT shouldn’t be looked at as a unified set of books. That’s how you end up with all kinds of serious errors in interpretation. The NT is a collection of 27 individual books with individual authors each with different beliefs, backgrounds, upbringings, and practices. We should treat it as such. It’s not about “letting the Bible explain itself”. It’s about letting each book explain itself.
Let me ask you this: If we take Matthew by itself without trying to harmonize it with other gospels, do you think that the disciples doubting would at least be a plausible interpretation?
> So it's easy to see how their heads are all in different places, and there are different degrees of understanding and digesting of what is going on.
Fair enough. I disagree with your conclusion, but it’s at least possible that that’s what Matthew is communicating here.