Your theory makes a lot of sense, although it also demonstrates why S4 annoys me so. It had some great individual moments, but the plotting makes the kind of sense that doesn't.
As far as Angel/Angelus, my opinion has always been that it's really everyone else, particularly Buffy, who wants to believe they are 2 different people. In Doppelgangland, Angel tries to tell her as much:
Willow: It's horrible! That's me as a vampire? I'm so evil and... skanky. And I think I'm kinda gay.
Buffy: Willow, just remember, a vampire's personality has nothing to do with the person it was.
Angel: Well, actually... (gets a look from Buffy) That's a good point.
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As far as Angel/Angelus, my opinion has always been that it's really everyone else, particularly Buffy, who wants to believe they are 2 different people. In Doppelgangland, Angel tries to tell her as much:
Willow: It's horrible! That's me as a vampire? I'm so evil and... skanky. And I think I'm kinda gay.
Buffy: Willow, just remember, a vampire's personality has nothing to do with the person it was.
Angel: Well, actually... (gets a look from Buffy) That's a good point.
:-D