A few years back I was reading a WIP that i've list track of.  Started in LA with Spike and Dana.  Had Spike traveling across country while Slayers were fighting each other.  Buffy and Dawn were in Canada?  Hints that the First hadn't really been beaten but was hiding inside Buffy.  Ring any bells?

It doesn't match all your criteria, as in the fact that Buffy isn't in Canada and 'William' is human (kind of) and has lost his memory after NFA, but there is a 'Buffy possessed by the First' story over on Elysian Fields - 'The One' by Chosen. It's a WIP but has recently been updated.

love that Spike Icon - NICE