You might be right, but if we continue to assume that Vanitas wasn't the one brought to the future, then YX must have known that a Vanitas was already planned to join the True Org XIII. And I think that someone might have already decided which versions YX was to collect from the past (I once read somewhere that Ansem explained everything to all the SoD) so I can definitely see Terranort being taken by him, especially since we don't know his full story yet. We have lots of moments when he might have been taken by YX, especially when he was brought out of the Keyblade Graveyard by the X-Blade's explosion.
Someone definitely could have explained everything to Young Xehanort, except that the Ansem who visited him was the Ansem from before Kingdom Hearts 1 (at least, I'm pretty sure that's the Ansem who visited him; if I am mistaken, please tell me). That means that Ansem would not have known what the future held yet, and, as far as I know, Ansem was the only one who contacted Young Xehanort before he set off on his mission. This means that, if someone did tell Young Xehanort who to collect and when and where to find them, it would have been sometime during his mission while he was in the middle of searching for his other selves.
As far as Terra-Xehanort is concerned, I think that, if Terra-Xehanort is going to be one of the seekers of darkness, he would have been the one that Master Xehanort was talking about in Dream Drop Distance. However, that would mean that Aqua isn't one of the seekers of darkness. The only way that she could be is if she becomes the thirteenth darkness, which would mean that Riku and King Mickey would have to find her later than I am guessing they do. This is all entirely possible, as I am expecting that I could be completely wrong. However, based off of the few out of context cutscenes that we have seen in trailers so far, I am guessing that she is a seeker of darkness and that she is the one Master Xehanort was talking about in Dream Drop Distance, meaning that Terra-Xehanort can't be one of them. However, like I said, it is entirely possible that I am completely wrong.
Edited by Dagesh Lene, 11 August 2018 - 06:49 PM.