I would only say that, while you're absolutely correct about the nuance of who is a victim versus perpetrator and touching on the cycles of abuse borne of only ever punishing, never seeking to help -
This also isn't the story for that. It is, very intentionally, a story seeking to reverse the usual way these things go, where the victim stays victimized, and the perpetrators--including the victims that are actively complicit--don't see consequences because the adults in the room are just backdrop noise, if not worse than useless and actively make things even harder for the victims (even if "trying to help" on rare occasions).
This is the answer to all the NTR doujin where the woman, even if she was seduced, still revels in the abuse and suffering of her partner at the hands of the antagonist, while everyone around the MC does nothing if they're even aware in the first place.
It's not seeking to be a critique on the genre or the problem presented, it's not meant to be a deep deconstruction of the problems involved in why people do these sorts of things to one another, and how that can be addressed and mitigated while the victims are given help.
It's just a story about revenge and the karmic backlash that comes for those who hurt the protagonist, and there's no time given, by design, to exploring whether some of the characters maybe should have been helped instead of punished at some point.
And regarding Miyuki - she's arguably the worst abuser of Eiji. She's his childhood friend, who knew him for over a decade. She was ostensibly his girlfriend. She chose to listen to her friends, against her better judgement, meet Kondo, get swept up by him, give everything to him, reject Eiji, corroborate the lie of Eiji abusing her, and every step of the way knew what she was doing was wrong, but she liked the sex and the feeling of immorality too much, and then she was worried about the backlash if she fessed up to it all and so she continued to hide it, just so certain that "Eiji still loves me, so he'll definitely forgive me".
At what point does she stop being a victim to Kondo? Sure he's an asshole abuser and manipulator, but she knew something was wrong from the start, and just kept going out of selfishness.
And she only vocally regretted it the moment she actually got caught. And in the context of this manga's message, her retribution is unceasing because even with Kondo, if Miyuki made one different choice somewhere along the way, Eiji doesn't get hurt like he does. And, because it's this story specifically, there's no "rehabilitation or mercy" because that's not the intent behind the premise.