Well, it wasn't such a severe separation that Arthur and Page don't see Cobb walking by from where they sit. Cobb may then walk into another adjacent room, but he's in the scene. Doesn't much matter, as the forgery scenes you pointed out require some deft acrobatics on my part to maintain my current version of my theory of the film.
The gaping plot hole I'm thinking of is ...
This is so much like a dream conundrum. But it's probably just a gaping plot hole. I don't know why movies are so riddled with them, despite years of preparation and constant attention.
Spoiler:
But also supporting my It's-All-a-Dream theory is the style of editing. Most movies jump from place-to-place, never showing the details of people getting in or out of vehicles, or the mundane routines of travel or interim time where no plot points happen. But I noticed early on that Nolan's editing style for this film was extreme in this regard, calling distinct attention to these jumps instead of adhering to the typical style for this sort of thing, which doesn't usually pull you out of a film by highlighting the editing.
So when the film specifically referenced the dream effect of never knowing how you got anywhere, but always just being plunked down in the middle of "scenes," I sensed a winking nod to the film's exaggerated editing style.
That style continued in much of the "real world" scenes, and was certainly as exaggerated in the final sequence (that was purposefully a mystery) as in any other dream sequence.
Of course, since it's set up to puzzle the audience as to whether Cobb ever wakes up at the end, I don't find it too much of a stretch to assume he's asleep at other times we assume he's awake.
As for whether he's asleep during the entire movie, I have to choose between the illogic of him dreaming of scenes he's not present for, and a gaping plot hole that negates his entire motivation in the film.
I don't remember my own dreams very often, or in great detail. Do people have dreams in which they are not a character, or in which they are missing from some scenes that feature other people??