Sam, that is an excellent interpretation of the course of events. I had not heard it presented that way and have done quite a bit of talking about FC myself. I think it's dead-on.
The point I always make to people on the other end of the film is that Norton is ALREADY CRAZY the minute the story begins (after the flashforward). Fincer gives us blips indicating Tyler SURFACING into Norton's awareness. This does not mean that Tyler doesn't already exist outside of Norton's awareness.
Lack of sleep, waking up in strange places...those are symptoms of "switching" into Tyler while being unconscious about it. He is working those jobs and living that life long before he meets Tyler "in person".
In short, Marla does not make him Tyler, insomnia does not make him crazy (crazy is giving him insomnia instead). Marla pushes his buttons more and helps bring Tyler forward as Norton deals with those new emotions, but we already know he has trouble dealing with love because of his reaction to the groups. The groups allow him to deal with his emotional issues and relieve him of the problems, thus he STOPS reverting into Tyler and SLEEPS.
But my feeling is that it's his own coldness of his career that finally pushes Tyler out. Note the sequence of scenes leading up to Tyler's first 2 full-on appearances - right after A) seeing the horrible wreck and B) telling someone about it. That ordering is not just chance, that's the last straw for him emotionally. It's another emotional detachment that he just can't take. He needs these emotional connections but can't overcome himself to make them (as Sam told us).
Marla made him see blips of Tyler, a faint hint on the horizon of his consciousness, but other stresses did that, office work, the doctor denying him medication, being asked to open up at group. Those are moments of extreme stress in his life and are thus tied to flashes of Tyler coming to the edge of his awareness.
I disagree- he only thinks he is. I thought the last chapter made it pretty clear that he is actually in a hospital, and believes the nursing staff are angels.
And thanks to Seth for bringing more great points to the table! I too agree that Norton is insane from the word go. Maybe we should cook up a 'Fight Club Compendium' or something!
And I'll add another vote in support of Iain's explanation of the book!
Go back and re-watch Marla's reactions to the narrator (especially when he goes into the schizo mode), it's brilliant! The person she sees as "Tyler/Narrator" and the person we see are pretty different, and yet the same. I love this film.