Yes, Lap, the old Xehanort forms cannot be destroyed in the post-DDD present. Time is immovable, as Young Xehanort tells Sora. Only I doubt that YMX, Ansem, and Xemnas disappear into their correct times when they are defeated, I believe that they cannot be defeated. If they were defeated in the present, then they would not have made it back to the past and would have (to the heroes) seemingly disappeared some time during BbS/KH1/2, leading inevitably to a very awkward level where all the members of the Organization and Sora search high and low for the missing final boss of KH2. In short, we know they must survive KH3, because they already have. We've seen the end of their lives, and it's not in KH3.
So no matter what happens in KH3, Ansem and Xemnas will not be destroyed. Something will interfere to save them. They can be wounded or smacked around a little, but they cannot be destroyed, because that would create a paradox. This could definitely lead to some interesting situations. If YMX, Ansem, or Xemnas sees the inevitable downfall of MX, then they will return to their times subconsciously knowing that they are destined for failure. Which is kinda cool. And depressing.
That being said, I'm curious to see how MX plans to reunite Team Xehanort. While YMX could theoretically pick everyone up again, he forgot his excursion to the future as soon as he returned to the past, so he has no conscious knowledge about time travel. I guess somebody will go back and tell him again? Would the Xehanort that goes back have to give up their body again? Or are they still covered by the "Give up one body, get time travel for life" clause?
