Just to add to your post it is a fact Xemnas remembers being Terra and MX:
Q9: Where did the Organization’s coat and mark come from?
A: Xemnas remembering his human years.
One year after BbS, Xehanort as well as five other apprentices toss aside their hearts. Then the Nobody Xemnas was born, and the Organization which he creates take a lot of things from his memories as a human. But as Xemnas had two people who he was, Terra and Master Xehanort, he takes from both of their memories.
The coat used by the Organization was something that Master Xehanort originally wore.
The Nobody mark used by the Organization is a similar shape to the mark Terra used.
Xemnas gives Organization members names with an X in them. This has something to do with Master Xehanort’s interest in the X-Blade.
So he definitely has Terra within but how much is what's left uncertain.
Ansem on the other hand has displayed that he fully knows who he is as well. His clothes, speech, mannerisms and even hand gestures are all MX.
However the biggest indicator was in DDD. It was Ansem who gave YX the power to time travel and sent him to find the others.
The very fact Ansem knew were
and when to travel shows he indeed remembers he is Xehanort.
After all he couldn't travel to Young Xehanort, MX at his youngest stage in his journey, if he didn't remember
being him.
The second indicator is YX's statement that Ansem is "Xehanort reduced to just a heart, the being your and your friends call "Ansem"".
YX doesn't refer to Ansem as being "Xehanort and Terra's heart" or as being their heartless, he specifically identifies him as "Xehanorts" heart.
The third is the opening scene of DDD. It depicts the moment the apprentices became heartless/nobodies. In this scene Apprentice Xehanort pulls out MX's keyblade.
He could only summon that weapon if either A) he remembers it or B) MX had taken control of him completely. In either scenario it ends with MX being the one manipulating him meaning he remembers, at least mostly, who he is at that point.
The fourth and final indicator however was the revelation in DDD that Ansem & Xemnas weren't following their own plans, but rather, following two halves of MX's grand scheme. Sure they had their own twists to it but it was still MX at the core.
They could only enact that plan if they remember having it and since it's MX's plan they could only enact it if they remember being him.
So there's no doubt AX, Ansem and Xemnas remember who they were just as there's no doubt that Xehanort ruled them and Xemnas for most of the time.
It is however left to wonder how much of any was divided. MX was obviously dominant but Xemnas had Terra traits though subtle.
Plus Ansem & Xemnas weren't born from one heart but three and if Xemnas remembers Terra then it's likely he remembers Eraqus explaining how he knows of the Chamber of Sleep.
Keep in mind LoD's "Castle Oblivion mode" is something only the "true successors" are privy too. It or the chamber isn't something Aqua & Eraqus are gonna tell anyone other then who they choose to succeed them.
Though we don't know why, LoD is very important so it's not info they'd tell regardless of the relationship to others.
So Eraqus seeping into Xemnas' makeup would explain his awareness of said chamber or even where to find CO.