Like I said, I watch it again to see for myself and Marla is definitely another figment of "Jack's" imagination. The REST, however, I don't buy. Such as, the house on Paper Street not being real (even though I DO believe it an abandoned house and nothing in there really works like the phone, or water, or electricity....I mean how can it?) Or Bob not being is suspect or a bit of a stretch, because there WAS indeed a scene where both Bob, "Jack", and Tyler were in the same basement (even though not all at the same time), and there's the death scene where all of his members started chanting Bob's name in honor of his death.... if Bob was imagined, then all the members must be imagined too (which very WELL could be) but I take it as a stretch.
Marla, on the other hand, IS NOT REAL. It, in fact, makes PERFECT SENSE as to the relevancy of her character and why Tyler always deemed her as a threat. This is why the film starts off with the introduction of Marla (well technically the introduction of Bob). Marla is the complete alternative to Tyler. The introduction of Marla was "Jack's" first alter-ego when he was constantly going to these different support groups to which he always came as a DIFFERENT person to begin with.... remember, he once called himself Cornelius in the group that Bob was in. Through those group was when Marla began to show and he IMMEDIATELY became disgusted by her because she's a reflection of HIMSELF. This was why Tyler was introduced afterwards. The moment of choice of which persona to embrace came when his condo blown up and he was to either call Marla or Tyler and he initially was about to call Marla but changed his mind and called Tyler.
Yet, throughout the movie, Marla kept trying to reenter into his subconsciousness. And "Jack" even stated how she sudden been "invading" his every move. "Jack" even stated he NEVER saw Tyler and Marla together, yet you ALWAYS saw Tyler as well as Marla interact with OTHER PEOPLE. The ONLY time you saw Tyler and Marla have a actual scene outright was she was overdosing and Tyler came to save her whereas "Jack" didn't want nothing to do with her, and that's the scene that was the most telling. From neither of them having mirror reflections to why "Jack" even transformed himself to Tyler to save Marla (which meant saving himself) to even Marla counting down to ten and Tyler miraculously made it from the abandoned house on Paper Street up to her hotel under 10 seconds flat to where she even stated in her drugged out state "damn, you're quick". They are all the same PERSON.
This movie's definitely "A Beautiful Mind" on steroids.