That is very interesting. I could see the Living Dead potion being possible if D hadn't just drank all that liquid in the cave. He said himself is was lethal, but not immediately. I think he was suffering it's effects when he was talking to Draco. And when he was saying 'Severus, please' to Snape, I didn't think he was begging for his life. I thought he was asking Snape to help him because he had drank that stuff. Also, I thought the portraits in headmaster's office are of dead headmaster's. If it were just previous hm, Dolores Umbridge would have a portait there.
But other than those two points, I think the theory has real merit.

Thanks for the link.