I honestly believe that Dumbledore is dead, but that whole polyjuice thing could be credible with some thinking.

Dumbledore could have been kidnapped, then someone else was forced by the imperious curse to drink polyjuice potion and act like Dumbledore would and do tons of other things like that. Voldemort could have wanted this so as to get a better chance to possibly kill Harry them or later, ya kno. So, then the person that was killed was someone else that was friends with Snape or something and he didn't know who it really was.

confusing, huh, well I understand it kinda.