I'm just gonna point out again Terranort's immediate lack of memory due to the destruction of the χ-blade. I mean, if that's what happens when all that power overwhelms you... who's to say that all the preparation in the universe will let you be all OH OKAY I'M BACK TO NORMAL after coming back.
And honestly, I don't think Braig (or anyone short of MX himself) is powerful enough to return after that. I think that's the main thing here.
No one else (in the category of villains) is powerful enough to return to existence (and in multiple copies, no less, as the Re:coded secret vid hints at) after having both their Heartless and Nobody destroyed.
Okay you're way off here.
We don't know why Terranort seemed so dazed and lacking memory before his fight with Aqua. For all we know it could be some weird thing between Terranort and MX's possible nobody that was created in BBS. But obviously Terranort's memory loss after the battle with Aqua was do to stabbing himself with the keyblade.
And about heartless and nobodies reforming into a whole being after their destruction, what you're saying is just silly. Nomura said a long time ago that if a heartless and nobody are destroyed in that order then they will become whole again. This goes for all heartless and nobodies. So obviously Xehanort is coming back, but it's just as easily possible for Braig to return, along with anyother member of the organization if Nomura sees fit. Heck I can prove Xehanort is not the only one who is returning as a whole being. It's already been hinted that Axel became whole again after his destruction so Lea is going to return as well.
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So like I said, it is equaly possible for the scene to take place in KH3D, and it is equaly possible for it to take place as a flashback to Xehanort's time as Ansem's apprentice. At this point we can't say for sure either way, so we must wait for new information that can confirm one way or another or simply wait until the game is out to confirm it.
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EDIT: @Blank Point: You don't die when you become a heartless and a nobody. Dying is completely different from becoming a heartless and nobody.
And yes obviously the members of organization XIII remember their past lives. Roxas couldn't remember his past life because Sora was a heartless for such a short time, Roxas didn't have enough time to get Sora's memories transferred to him. Roxas is the exception the rules, all other members of the organization remember their past lives as whole beings.
Now it's a fair question to ask if a person remembers their time as a nobody once they become whole again but I personally believe they would remember their time as a nobody because it seems like if a nobody remembers their past as a whole being then a whole being that was once a nobody should remember their time as a nobody.