In any case, it comes back to "actively attempting to improve position". If you are defending the choke and/or attempting to escape the back mount (often times one needs to defend the choke before they can escape), you are actively trying to improve your position. It's not passive like turtle is.

I don't see how or why this is true. Effective defensive in turtle is just made exceptionally easy by the quirks of the position and the other contingencies in the Unified Rules. It's very hard to choke someone in the turtle position. The floor provides excellent defense to strikes to the face and front of the head. With one's hands in the appropriate position, it's hard to large hard shots that aren't illegal. (And this is why I said, in my opening post, that the turtle position is a stupid position made effective only by illegality of strikes to the back of the head.)

You're talking a lot about the "potential for significant damage." If the potential is there, then the top-position fighter should endeavor to make the damage actual rather than potential. And in the course of aiming to make it actual (i.e., actually hitting the other guy hard in a non-theatrical way), the ref would then have grounds to stop the fight (i.e., intervening to stop actual damage from eventuating).