Im with you in this... Im not touching those spoilers with a ten foot stick, I prefer to enjoy the story inside the game and be surprised....that said I do like to speculate about possible outcomes, a little fan theory to get the creative juices flowing and maaaaaaaybe inspire Victor to do something cool with those ideas.
In my opinion I really like Cecilia as an antagonist, her being the final boss of the story or at least a big scary looming threat that can (and will) surprise you in the worst possible moment is pretty exciting, I want to see more of her yandere antics before she gets finally caught, after all, those flashbacks presented her as real threat and I don't wanna see her "defeated" until much later in the story.
And speaking about her resolution Im focusing on an hypotetical redemption path since I don't think anybody is beyond redemption I think Cecila will be arrested and sent to prison after having a mental breakdown in her final confrontation with Aurum and his gang, maybe if you choose to try to reason with her during other earlier encounters you will plant the seed of doubt and eventually you will be able to punch some sense into her and make her realize she has became a monster even worse than her sister making her finally surrender (after a good bunch of action packed/adrenaline pumping moments of course).
But is already too late, there is already too much blood in her hands to be left free, poor Nook won't be able to swim again without her arms. So she needs to pay for her crimes and go to prison, but you also noticed that she is terrified, not for the prison but for herself, she just realized she had monster in her head that fills her obsession with you and wisper terrible thoughts when sees you hanging out with other girls, now she can see it but is not like she can just make those voices dissapear with a snap, she is afraid she might become a yandere again, after all, deep mental trauma can't realistically be healed with one heart touching conversation inside an old manor on fire. So Aurum decides that he needs to help Cecilia, so he decides he will visit her in the prison every time he can, to talk to her, calm her and try help her fight her demons, he knows he will have to go alone, he can't ask any of the other girls to come with him, after all that happened, some of them hate her guts, and most of them are terribly afraid of her, and with good reason, is not fair for them, you must go alone, you are worried that if you decide to visit her you might end up making her dependent on you for emotional support, being the only light in her life again could fuel her yandere self, but you just can't sit around and leave her alone in a tiny cell, trapped with a demon she can't just stab with a knife, you became terribly afraid she might try to end her life in order to silence those voices, you must do something, even if is not the best solution, is a solution that will keep her alive. You get ready to visit her on the prison after your wounds healed but before you leave the hospital you notice someone waiting outside, is Gillie!, her first victim, the person that should be the most afraid of her is there, and just as she saved you when you were alone and with no ties to the city or the college by introducing you to her friends, she wanted to help Cecilia and become her friend, that's what she really needed, friends, she doesn't need to rely on only one person for emotional stability, when you have friends that support you they can share the load of your burdens, they can all help you fight your demons and you can help them in return, that's what Cecile really needed.
And that's how after a few months of constant visits from Aurum and Gillie, gradually all the girls, one by one, began to join in to the visits as well, and before you noticed it, Cecile managed to become real friends with all the girls and the voices inside Cecilia's head gradually dissipated because she didn't needed to obsess with the one light in her life anymore, there was a whole sky filled with lights now.
lol, I didn't expected to write all that, lol, I was trying to dive a little bit into the psychology of a yandere, and though, usually when people get super obssesed with someone is due to an inate fear of losing it, and that makes sense, Cecile life's was hell with her abusive sister and Aurum was the one who rescued her, he was the only light in her life at her lowest moment, so I though it would be interesting attack that angle, instead of being Aurum the savior I think it would be more appropriate to give her a group of friends that will help her overcome her trauma, of course with Aurum there as well, I also wanted Gillie to be the one who initiated the whole thing because since Gillie was her first victim in the game, it would make the scene more impactful seeing a person who has all the right to hate her being the first one to come to her rescue, and thematically ties amazingly well with how Aurum's life changed when Gillie introduced him to her friends and now she can do that again to save Cecilia.
But hey, that's just a theory, A FAAAAAAAN THEORY, thanks for reading!