Just had a thought about the Dandelions who ended up in the 'modern' Kingdom Hearts time with no memories in light of Dark Road.
So thinking about it, it seems like moving forward in time doesn't mean you HAVE to lose your memories, but out of the five Union X characters that get shot into the future (assuming here that Subject X is Skuld), why did only Brain keep his memories?
But let's assume that Xehanort actually is the waypoint for the other four. When Brain arrives in Scala, his identity has been passed down from Ephemer, and people know who and what to look for. His name and role in society have been preserved in the memories that are his waypoint.
But Xehanort doesn't have the first clue who any of the friends in 'his' memories are.
He doesn't know their names or their roles in society at all. He only knows their faces, what they feel like as people, and that they are all friends and all have keys. Four friends and a key is the only thing Subject X remembers. And Xehanort does collect all four even when his own memories are a trash fire from taking over Terra.
What if none of them remember their true identities, because Xehanort doesn't know them?