Ruth, your theories are fascinating. Although I've loved both Life on Mars and Ashes to Ashes, I have no real idea what's going on in them. Reading your theories made it even more exciting!
As to what Keats did to Viv, it was obvious Viv was really scared of Keats, and the only thing I could think was that isn't there a place on the neck that can kill you if it's pressed in a certain way? I wondered if that was what he had done.