I'd like to think that Sekai is not *forcing* these thoughts, dreams or emotions onto Sensei. I feel like it would really cheapen his struggle if this can all just be blamed on external interference. At that point Sensei becoming a better or worse person would seem to depend more on the efficacy and/or availability of an exorcism than his own choices and character development. Better that she draws to the forefront thoughts and feelings Sensei may have forgotten or repressed - which he is quite prone to do, as you point out. (Unless everything in this game is just a representation of Sensei's mind or something, making it a moot point.)
I do wonder what the motivation behind this propogation of her philosophy is. Uta reversing to her younger self and then turning into Maya as part of Sekai's attempt to influence Sensei seems to suggest that she approves very much of the past younger Maya/Sensei relationship and considers it to be some kind of high point with regards to Sensei's acceptance of her beliefs. It may be that Sensei deciding to do to Maya what she did to him would appear like validation to her, that he approved of what she did since he would do so to others as well. "You hate what you think, yet you always think of what you hate." Perhaps applicable to herself as well as Sensei? Perhaps she wasn't quite as bad as Nodoka, and this philosophy she engineered to present herself as being above morality was in some part intended to help her reason her way out of that bit of lingering guilt in the back of her mind over what she did. Making her victim, of all people, accept and internalize her justification for why she abused him would be the ultimate victory in that case. Well, all this is baseless speculation, really.
As for Nodoka, I don't think there was ever much hope for her to not be awful, since IMO she's clearly a psychopath. She was only ever going to behave if by some kind of calculus she arrived at the conclusion that comforming to all social norms would be more beneficial to her than breaking them when she can get away with it. But considering her narcissistic and sadistic streaks, I think it's unlikely she would have made that choice, even without Sekai. On the other hand, if it is the sort of material that someone like Nodoka would find agreeable to the point of integrating it into her self-image, that probably says enough about the content. Still, I really wonder what she could've written, in the form of poetry, that would be so disturbing as to alarm Wakana to such an extent.
As a side note, there's something funny about Sekai's appearance in this event for me, with her just hiding behind a tree spouting verbal abuse and horrible suggestions at Sensei. It's like the inverse of that "Playing Cyrano" trope where a friend is hiding in the bushes and giving advice on what to say to a girl.
