What confuses me with the whole thing is, ok, say it's just plain old YMX. So... YMX is one of the 13 darknesses? If that's the case, why didn't Xehanort just go back in time like 12 times and get a bunch of random Xehanorts from the past as opposed to going through all this trouble. Like get YMX, YMX a week later, and after that, and so on lol.
YMX also says that when you time travel you can only go in one direction with the flow of time.
So Ansem SoD recruiting YMX to do this task is the easiest, since he is the earliest Xehanort, as said by himself.
YMX goes forward in time and finds each Xehanort, although he doesn't go forward in time with each individual one of them (Since he wouldn't be able to go back in time after that).
One thing though I've been wondering, is that the MX in DDD isn't the one from BBS. YMX says MX is the latest form of Xehanort, and BBS MX is chronologically before Ansem SoD and Xemnas. So this MX is the Xehanort that reformed after Xemnas and Ansem SoD were vanquished then right?
But that should be AX....right? Not according to Yen Sid though I know.....strange really.