Wow, that is thinking outside the box, but there are ways it can be disproved I'm afraid. First of all, Malfoy was already pale, but that was because of the job he had to do for Voldemort, and I don't really see where exactly you're going with this, what does this have to do with either Harry or Dumbledore being a horcrux? And you might have a point about Dumbledore being already dead while he was still drinking the potion, the only thing though is that then how could he have talked and at least moved a little? And you can't say he's an Inferi, that wouldn't make any sense! Ok, I think that should do it......for now.....