I noticed something. During the scene with Braig and Sora, where Sora is talking about his friends being his power, Braig seems to lose a bit of confidence in his Master. It looked a little like he tried to fight back, scrunching his face up in pain for a split second before Xehanort's will overpowered his again. I believe that if they can break Isa's loyalty to MX, by getting him to remember his friendship with Lea, he can be freed. Same with any of the Darknesses, possibly including Terra in there. Even Xehanort's powerful darkness cannot match the power of friendship. I'm pretty sure that will be the lesson in KH3. But for now, Isa's under Xehanort's command. We'll probably fight him at least once in 3.