There is no hard fact to state that the two were seperate, name changes happen allot in myth. It can also be seen that Balor gave them new names when they came into his service, and changed Bahl'al to The Watcher. Despite your argument there is no concrete evidence to prove that Bahl'al is not the Watcher, your points can be taken a different way.
Having observed the Myth story community for two years i have come to see Myth canon akin to the English language,when a word becomes widely used enough it is accepted into the Language, through this way meanings of words can be changed over time, and by false assumptions. Likewise in myth, if everyone believes something they it becomes true. A more obvious example is the A/B spirit cycle, which when observed closely has no real evidence to support it, however you'll have allot of difficulty trying to stop people believing in it.
Whether or not Bahl'al is the Watcher is a matter of personal choice, as there is nothing truly concrete to support either idea, whatever their advocates claim.
-zeph
