I was reading something once that Stephen King had written about how he first created a map of Castle Rock (and I guess surrounding areas) and then his characters had somewhere to go, and to be, and all of the plot had somewhere to happen. And I think he added to it, as time went by. But it was a way to keep the characters where they were supposed to be when he would get wild hairs.
Your map would be interesting to see.
I love the idea of this story, btw. Wish I’D come up with it…;)