I agree with a lot of what you said, but I still think Tommy is a Do-er. The scene at the restaurant with Judd Apatow is a great example. His whole idea is 'I have to prove myself, I have to go up to that guy and introduce myself, and then perform a monologue loudly so everyone knows I can act'. That is very much a Do-er move -- I have to DO something here. And, also, look at how he reunites with Greg toward the end -- he goes along to one of Greg's plays just to meet him outside. It's all external with Tommy.
Whereas Greg is very internal -- he wants to be an actor, but he doesn't have the confidence and balls that Tommy has to do anything about it. Look at the scene in the diner -- he's so reluctant to perform alongside Tommy, he'd rather just adapt to whatever is around him. And then when he's faced with the decision between doing Malcolm or finishing The Room, his first instinct isn't to act, but to consider everything.
I definitely think it's an upper-right story. 'Making a Movie' seems to be the goal, and I agree entirely on the relationship in Psychology. Lots of manipulation going on either side in the RS, while the OS is handling the actual production side of things -- buying cameras; dealing with fainting actresses; uncomfortable sex scenes; having a cameraguy to record everything going on.
My argument for MC Greg in Impulsive Responses would probably be his inability to go with his impulses in regards to acting. His teacher chastises him for being too withdrawn and held back. He doesn't want to do anything impulsively -- he even stops to think about going to LA, even though he knows it would help. Tommy's influence in Progress is targeted at moving Greg's career along faster than he can stop to think ("Why you don't just move to LA?" "You get agent now, you big star.", etc.). It's been a couple days since I've seen it, but that's where I am at the moment.
If I had to go with my gut, I think it's an OS Issue of Experience. That quad of elements feels very strong to me (in my mind, one specifically is undeniably the Problem element), and the theme of 'gaining familiarity' with Tommy and the way he works rings true for many of these characters. The counter is Skill, which... yeah.
As to the ending, I felt it was a happy ending. I mentioned to my friend that if you think about the logistics of the reality of it, it's kind of sad because this guy will be forever linked to this bad movie. But in the context of what The Disaster Artist offers, it feels like a happy ending. He gave up a potential acting career to finish this movie with his friend, people liked it, and he's okay with all of that.