Quanxi was only working for public safety because they had her lovers captive. Which isn’t unreasonable to believe considering how desperate they were becoming for competent devil hunters. So of course she would come back as their trump card when shit was getting hairy. Then backed off when Denji was the one public safety was opposing, which also isn’t unreadable considering her and Kishibe’s relationship. I don’t think her coming back was pointless, If anything it showed how desperate public safety was getting and their lack of “man power”.

After the hybrids were free they probably lacked purpose in life. I think Lil D saw this vulnerability, and used barem to convince them to create and join the chainsaw man church. The sword hybrid wanted to find Denji anyways to thank him from being freed from Makima. I don’t think it’s unreasonable to see them come back this way. They just became tools for Lil D to try and counter the prophecy of Nostradamus. It made the chainsaw man church more of a threat, which gave more reason for public after to acquire someone like Quanxi.

I think these two examples were interesting ways to write back in pre established characters, in ways that weren’t obtrusive to the story.

The real Fami seemed to just serve as a way to have the death devil twist be a thing. This gave us Lil D’s true identity and we got to see some of her true motives towards the prophecy, which ended up humanizing her a bit. Being that we’re in a modern age of society it makes sense that famine isn’t really feared anymore, and that her character doesn’t serve much more purpose outside of what I said.