So
I think Dumbledore really is dead. Why? I wouldn't be surprised if there was a plan but it all went wrong.
Faked-death-gone-wrong scenario one: Snape and Dumbledore had planned to fake Dumbledore's death, but the Unbreakable vow made Snape actually kill him. Dumbledore didn't see this coming until Snape walked in the room and it was too late. Between this and Potter's presence, Snape cracked.
Faked-death-gone-wrong scenario two: Snape and Dumbledore had planned to fake Dumbledore's death, only Snape was going to really go through with it when the time came. Dumbledore didn't see this coming until Snape walked in the room and it was obvious.