I didn't think that anyone believed that number theory was consistent and complete. [0] https://en.wikipedia.org/wiki/Peano_axioms
That's nitpicky and missing the point. If the only problem with a theory is that it's been reduced Goedel-style to an axiomatic base, then most people would argue we "understand it". Needless to say 3x+1 does not follow in any kind of obvious way from Peano. It's emergent behavior that flies in the face of axiomatic understanding. And that's why it's interesting.
I don't follow. How are the rules and outcome of 3x+1 any different in a qualitative way than the Game of Life?