Interesting ideas. I always thought the gold eyes and changes to Isa's appearance was because of his berserk side, that possibly when he lost his heart he was somewhat trapped between states. For the x I figured he might have gotten it when he lost his heart maybe protecting Lea, and that the marks on Lea's face were tear marks,him crying over his friend then dying as well. By watching the MF I thought Xemnas honestly with the way he fights and how he can call upon the attacks of the other members as he did in Kh2. I do agree with Vanitas falling into another heart, I thought Riku though with how he fights and looks. Honestly who knows as they have soo many plot holes and just seem to be throwing things at times. You do have an interesting idea though, something to consider as it is possible.