Thank you for the translation! This is getting interesting. I can kind of see that Ai's obsession into writing/finishing the novel might lead to some sort of toxic co-dependency between the two and put her in grave danger, there's also the fact that she currently has a few plastic buttons inside her body after chapter 2's ending. Seems like the cop has caught on to something weird as well.
I have to wonder though, at this point it's pretty obvious that Miu probably took over the identity of the "real Miu"' who used to live there, given that their elderly neighbour mentioned how she seemed "different", but it seems that she's able to deceive people's eyes despite having an uncanny appearance. Are people simply not able to perceive her face as the readers see it? I can't help but feel like something about this is a red herring.
There are instances of the plot being shown as novel texts written by Ai, and she added in a lot of supernatural elements to it (there's no doubt the beginning of chapter 1 was something she probably wrote), so this story might be a retelling done by Ai?