1) OOC behavior is subjective enough that, as you've said, I've read fics where I would call Draco and Harry OOC which still worked. (I've read other fics which people thought were canonical that didn't work for me). But if it becomes inconsistent within the story itself and no reason is given, that destroys the fic's universe, never mind congruence with the canon.

3) Yeah, especially with new powers. If the character really can transform into his Animagus form to escape a prison cell, why didn't he do it earlier when someone else was holding him captive?