This would have been the twist of the twists. I don't think it would have been very well liked though.
I don't think you can do something like that to the fans. I for one wouldn't have liked it.
I'd like that. Scratch that. I'd LOVE that.
Why? Because it makes sense.
Buffy is an incoherent mess, nothing makes sense in this show, NOTHING. The lore doesn't make sense, the magic doesn't make sense, characters don't make sense, the world makes no sense at all, OK? And I still love it because of attractive people, funny puns and those rocket launcher/hammer to the face moments and don't you judge me ,)
Buffy being mentally ill person? It makes perfect sense.