All the theories of the Watcher not actually being killed are imo remotely possible because there is no evidence that says it is false.
Except of course for the whole entire level dedicated to killing that man.
For some reason, I doubt very much that Bungie would put in a level with an objective of assassinating someone when in fact you didn't kill him at all. And then never tell you that you didn't actually kill him.
The bottom line in my mind was that the man was disabled by the ambush, and killed by a berserk's claymore on that level.
To assume that it wasn't the Watcher on that level would mean Alric sprung a trap on something else. A decoy? Some guy dressed up like him? Something that was already a statue?
Please. Alric is not a fool.
-Welly