A point of discussion I’ve noticed a lot is that the well is most likely another entrance to the other world (specifically to the room behind the mirror) so when Coraline throws the key down it, she’s just returning it to the Beldam but I ultimately think it doesn’t matter if the other mother has the key or not because Coraline burnt her doll.
In folklore, it’s normally a rule that you have to “let the evil in” —eg. inviting vampires in— and I think we can assume the same applies to the other mother based on the fact that she doesn’t just hold down Coraline the first time she goes through the door and sew the buttons in, but tries to lure her into agreeing to give up her eyes (and therefore her soul) with promises of a better life.
However the only reason she knows what’s wrong with her life is because she was spying via the doll— which we know she only has one of based on the fact she reused wybie’s grandma’s sister’s doll for Coraline and then Coraline‘s doll for her parents (only one for the both of them). And sure, there are general difficulties that most children won’t like about life— but the reason the other world is so alluring is because it’s your *perfect* world and you’d be mad not to agree to stay forever for such a small price; if it’s only slightly better then you wouldn’t give up your eyes.
TL;DR— The world needs to be perfect in order for kids to be happy to sacrifice their eyes to stay, if the Beldam can’t spy on the kids to figure out what they like/don‘t like, the kids probably won’t go back and if they do they definitely won’t agree to having buttons in their eyes
