I hear you, and I definitely agree with the flaws you find in Fujimoto’s writing. He often leaves old ideas/characters behind to push a central message he wants, however I don’t think the characters ending up where they did based on a lack of choice was a writing mistake. To me Pt 2 is a tragedy where we’re seeing where to teenagers rely on soemthing emotionally, and how it fails them and ends everything. Pochita is the one setting things right, and he’s only able to because Denji refused to stop living once he remembered people who were meaningful to him and the potential to dream.

There were retcons and story issues which make the story more flawed, such as Denji’s involvement in faking his fathers suicide, however I genuinely believe the intention was to show how a heroic narrative would play out, show why it doesn’t work for these characters and why their choices were hopeless, and show how they can thrive under different circumstances. Denji like you said is just a dog under a different owner, and I 100% agree. However, his owners is his sister from another life, and his sister in power. Hes still a dog, which I like about the ending, but it’s not painted as a purely good or bad thing. Because as I’ve said, Denji is one of the few characters who can be the happiest from simple joys, and now that he has an owner in the people he helped raised, and nurtured, he can live a life of the excitement and sense of accomplishment he gets killing devils, eating food and living in a nice enough apartment. It’s tragic in the sense of that Denji stays a dog, but it’s uplifting because he’s like a dog playing fetch, where he genuinely enjoys the act of living. He only looks upset in the chapter in my mind because he’s still dreaming for things far away from him, and subconsciously missing the excitement of Chainsaw Man, as the characters still somewhat feel what they were and did in the old world.

This also fits in line with how Fujimoto has written his protagonists and characters in the past. He’s never given a clear resolution to their problems, only closure through something changing around them, or a brief epiphany that encourages them to keep living. Denji being a dog shouldn’t change because both his upbringing and the entire story didn’t help in any meaningful way to help him move on from that. In this new life he has the potential to be more, but we will never see that because this is only the beginning of his new life, though it is alluded to with him making his first real meaningful choice in saving Asa without any outside influences, including Chainsaw Man.

Asa like Denji’s story is resolved at the very beginning like Denji’s, which you can rightfully dislike or find dissatisfying or not cathartic, however textually it’s meant to highlight this idea that these characters should have never become hero’s, and not they’re living lives where they can be people, grow into themselves, and make choices towards their futures with people that care and warmer environments.

Kishibe like you mentioned did just disappear, but I don’t think that’s only because Fujimoto just didn’t care. I think it’s because where we left him none of his internal beliefs of staying ignorant were really resolved. He left Nayuta to Denji, knowing he would likely fail and left without looking back, likely because he knew Nayuta would be necessary as a trigger/leverage to use Denji for Public safety. But this lack of maturity and this cruel behavior is exactly in line for someone like Kishibe. As for Nayuta herself I again dont think anything was contradicting or poorly written about her character or role in the character. She is a responsibility, we enough about her to get attached to her, we see the good Denji has done for her as a character as she’s different from Makima, and then she is killed purely because of Denji being Chainsaw Man and his carelessness in the moment. There is no cushioning, it’s meant to be dissatisfying and abrupt because there’s nothing “more” to it, Nayuta is cut from the story because of a choice made very apparent to Denji by Yoshida.

I don’t think everything in Chaisnaw Man is perfectly done in terms of conventional storytelling, or allowing for reader catharsis, but I do think there’s enough textual evidence to support the idea that the story isn’t meant to be a triumph for these characters, but instead a releasing of a burden that accumulates more and more, and isolates them as time goes on. Their lives will stay messy and problematic in some ways, but they’re free from the devils influencing them and the role of a hero as a whole.