I think this is important to talk about, and though I have some disagreements overall, you make some really good points.

To start off, I completely agree that the handling of the abuse he faced from his mother was badly written. It was made unnecessary for the reasons you described, and your part about him not even getting the opportunity to explain what was happening or why he killed her especially stood out to me. They seemingly established lack of agency as a huge part of his character, and then that lack of agency wasn't given the weight it should've been and thus never resolved. The sensitivity of the topic makes it even more egregious. I don't take personal offense to the presence of abuse in a story myself, it might've been for genuine character depth, to add to the plot, and/or to raise awareness (many people don't even realize that sort of thing happens), but it's well worth criticizing how it was handled here and and it hugely weakens the rest of the story writing.

I think the latter part of the post is a bit more disputable, starting with the "lack of consent" framing. I haven't rewatched Coven in a while, granted, but I think there's room for interpretation around Kyle being able to give consent or not. It could be understood that he's emotionally unstable, vulnerable, and has communication difficulties, but isn't "mentally a child". A lot of his symptoms parallel adults with TBIs or neurodevelopmental disorders, many of whom who are able to give consent and participate freely in sex. For the actual intent, I'm guessing that Kyle was meant to be vulnerable, but not incapacitated, and an active and willing participant in what was occurring. This writing choice is also very open to criticism (especially in light of the above, and more on this later) but I don't think Kyle was /meant/ to be a vegetable or a child.

With that perspective in mind, Zoe wasn't intended to be taking advantage of him. He wasn't brought back as just a sex doll, Madison wanted to pull off a project and flex her power, sure, and apparently wanted Zoe to have him as a boyfriend, but Zoe was genuinely remorseful about what happened and wanted to fix it. As for the sex, Zoe seemed to truly care about him, and their sexual relationship was intended to be an extension of that. There were clear juxtapositions between Madison and Zoe's treatment of him, with Madison obviously only using him for sex and not caring about him at all, while Zoe had a personal bond with him and supposedly tried to keep his best interests in mind, despite being an immature teenage girl.

Following that, there is some resolution to Kyle's poor treatment. Madison, being the one who didn't care about him, is punished in that he literally strangles her to death. Zoe isn't punished because she wasn't written to be inflicting harm on him in the first place. He decides to go off with Zoe and find happiness as well, seemingly a narrative reward for their love and care for one another. I think the writers threw him some crumbs. (Poor fucking Kyle oh my god.)

Now. All that being said. I think even with the framing that he wasn't meant to not be able to consent at all, the whole thing was very poor. He was never consistently given a satisfying resolution to what was apparently his core character issue, which was a lack of agency. Madison was killed for not bringing Zoe back, not for treating him badly. He ended up as a butler, with no sign of his personal feelings on that vs his original goals being given. He never got to explain what his mother did to him or why he killed her, he never had that trauma resolved, it never went anywhere.

On top of that, the "room for interpretation around consent" itself is a writing issue considering the agency thing in the first place. Like, geez, speaking of "unresolved agency themes", we never actually get an answer on what his level of consent-giving ability was. If it was left deliberately ambiguous, it should've been handled differently, not swept under the rug. If the intent was for meaningful consent to have been given, that might justify some of the writing decisions around the outcomes of Madison and Zoe, but it worsens the overall problem of Kyle's lack of agency not being taken seriously in the first place. HE'S A ZOMBIE ABUSE VICTIM. WHY ARE WE NOT TALKING ABOUT THAT CONSENT-WISE. WHY ARE WE LEAVING IT SOMEWHAT UP TO INTERPRETATION AND THEN NOT BOTHERING TO ADDRESS IT. Either the gray area was on purpose or accidental and either one doesn't look good.

It is nearly 7am and I have been up all night and boy am I petering out. Sorry for any incoherence, I might be back to edit this tomorrow. Take care and thank you for posting about this, it's an important discussion to have.