i do believe this is true the cutscence with xemnas and roxas where he says "i've been to see him he looks alot like you"
well i allways thought this meant sora but no.
he meant ventus/ven.
the room of sleep from kh2 final mix has aquas armour in there. xemnas sat down and addressed her as friend.
this is because xemnas is xhanorts nobody and xhanort i believe is terra whos memories must have been wiped after defeting master xhanort but all he could remember was xhanorts name but not his own.
so he took xhanort as a name and was found by ansem then he turned into a heartless and xemnas came into play.
so xemnas knows aqua and ven he has found aqua but is now looking for ven.
who is in the room of awankening. in castle oblivion hence why xemnas sent 6! members of org 13 in there.
also the scene where xigbar and zexion are talkin (final mix +) they mention all of this.
and so xemnas found ven and then all three frinds were together.
but remember when sora became a heartless? by finding kiari he became normal again. so the heartless was gone but the nobody (roxas) stayed.
just the same with xemnas.
his nobody stay but because he was back with his friends the terra we all know came back. hence why we fight him in final mix +.