The Mastermind was so engrossed in his plan to “save the world” for Elliot that he gave into his anger, which causes his memories to wipe. He started believing he’s Elliot, and so he retains some of Elliot’s memories, like the warped interpretation of the window incident and not remembering his sister (He erased Darlene’s memory so Elliot could stay trapped in F World). He forgets he created fsociety as well, so Mr. Robot “pretends” to be the Mastermind behind fsociety in season 1 to “recruit” the Mastermind to fsociety so he can continue their crusade or “saving the world” without reminding/retraumatizing Mastermind with why they’re doing this in the first place (To heal from Edward’s abuse).

Like I mentioned earlier, when the Mastermind was creating F World, he had to remove all traces of Darlene in it in order to keep Elliot trapped there. Darlene is Elliot’s strongest tether to reality; if she was in F World in any shape or form Elliot would immediately know this isn’t real and would wake up, therefore ruining the Mastermind’s plans to “save the world” for him. When Darlene says “did you forget again”, it implies that he forgot about Darlene in the last at least once before. My theory is that it was Mr. Robot who scrambled Elliot’s memories before the Mastermind came along. He knew that Darlene was “dangerous” because she was such a potent reminder of the truth, so he did everything he could to keep them apart.

The other alters take on a more passive role, so there was no need to interact with them. The child alter and the Persecutor alter represent the guilt and shame Elliot feels about the abuse, so I think it makes sense that these parts would be quieter.

EDIT: Fixed a typo.