okay first,there is no evidance to say that the watcher ever went by the name bahl'al, though that does not mean that it was not him.
what i found from the journal of silvermines, "looking for the arm of the second or third most powerful sorceror in living memory" where have we heard this before?
also and this is really my point, "Rather like knowing his true name, except better."
maybe the watcher's true name is bahl'al, but he is never referred to by it, because only he would know it. the flavor texts would have it because they want to let you understand who bahl'al is. before he was the watcher, bahl'al went by a different name(before balor)....
thats right Mad Goat of the Fens!
Gothmog