I think you're going pretty good there. Those are all facts no doubt about it so no one can say that your theory is lacking a foundation.
antiformkeyblade, that can be a possible way of erasing memories.
In fact, I'll add to your theory, With a bit of my own.
Say the 'Chasers' as we refer to them now, wanted to wipe Xehanorts memory.
What better way than to remove the heart?
We shall use the Roxas/Sora case.
Roxas did not retain the memories of Sora because Sora was returned so quickly.
If the chasers took Xehanorts heart, and instead of letting it become a heartless, they simply stuffed it into his nobody.
Guess what? The heart didn't, like in Sora's case, become a heartless. So suppose they can hold the heart until the nobody gains all the memories. Then, as in the Roxas/Sora case, they send the heart back to the body, the heart has no knowledge of what the nobody knows now, and the nobody's experiences aren't transfered, therefore the new complete version doesn't have any memories at all.
Thus it might explain why xemnas only has memories of being ansem's apprentice. It might also explain why Xehanorts heart didn't turn into a heartless when it was separated, it could have been used to not being in its body since it would have been the second time.
However there is one loophole or something that hasn't been explained yet, and I don't want to hear anything about it, because noone alive besides Nomura could explain it.
How did Xemnas AND his counterpart Xehanorts heart BOTH retain the memories of apprenticeship? that is where the true answer lies.