Well, i will answer many questions:
1-Young Master Xehanort is a young master xehanort from another timeline. He was visited by Ansem SoD BEFORE KHI (the Ansem from the original timeline) and the heartless give him the power to control time and travel through it (Ansem possessed that power). His mission was to gather possibles new vessels for parts of Xehanort's hearts, so he isn't MX, he's another incarnation of him. He fight Ven, Terra and Aqua in Birth By Sleep during one of his travels. His mission in DDD is to make Sora the last vessel for Xehanort's heart (his older counterpart in this timeline).
2-Ansem SoD and Xemnas in DDD are themselves from another timeline. If they were from the original timeline, Xehanort will never return, since if they never were destroyed in the original timeline, Xehanort will never resurrect.
I reviewed my memories about Ansem's plan in KH I and the only thing that i don't understand is why Maleficent and him used the Seven hearts and the Dark Keyblade to REVEAL the keyhole. I can see that huge portal from the entrance to the Grand Hall xD.
Allow me to further expand on this.
In real science there is a theory regarding the whole of physical reality. So i'll try to not make this complicated but here it goes:
As you know we exist in a universe. However besides our universe there are countless universes just like ours, governed by the same natural laws. In each of these universes there is another version of us that exist there. In this universe I am me, in another universe there is a me who is slightly different. For example in one universe there is a skinny me, in another there is fat me. In another universe I am girl instead of a boy. In another universe i'm evil. These other universes and other versions of me will all take different choices that leads down every possibility there is for what could happen to me. In other words in some universes the version of me in those universes is already dead.
Now all these alternate universes are tied together in what is known as a multiverse, which is the sum total of our reality.
However there is something beyond that known as the Omniverse. The Omniverse is the grand sum of all physical reality. Countless Multiverses exist in the Omniverse and each Multiverse contains countless alternate universes. So while in our multiverse there are alternate universes and alternate us in each universe, in another multiverse there is entirely different reality.
For example there may be a Marvel Comics Multiverse governed by different natural laws than we are. Within that Marvel Comics multiverse there are alternate universes. In one of those universes is a spider man and in another alternate universe is also spider man but different from everyother spider man in every other universe within that multiverse.
So now if we look at KH, there would be a KH Multiverse somewhere out there and within that multiverse are alternate KH universes. Now here is where time travel comes. One theory of time travel is that if you went back in time and changed the past then instead of affecting your future, you would instead cause time to split at the very moment you changed the past thus creating an alternate future/alternate timeline, and therefore creating a new alternate universe.
So YMX went back in time and took XH and Xemnas out of the past and into the present, but instead of affecting the main KH timeline, it instead caused a new timeline to be created due to YMX meddling in the past, an alternate timeline where Xemnas and XH were never killed and thus MX was never reformed. But the Xemnas and XH from that timeline were brought into YMX timeline which is the main KH timeline, to the present of that timeline where the Xemnas and XH of that timeline were destroyed and MX reformed.
Okay that came out to be alot more complicated than I intended.
