What was real and what was fake?

The Corpse (in the rug)

It was likely just an empty rug with no body in it, or Trevor dumps his own body in the river, only to come out and perceive his death from another one of his personalities.

Ivan (the Antagonist)

Completely fake. He never physically existed. Ivan was purely a projection of Trevor's guilt toward the car accident. Under stress, Trevor's mind found it easier to pin his guilt on an imaginary personality than own up to it himself.

Miller (the Machine Accident)

Somewhat real. The accident injuries were likely minimal if they even happened at all (explaining why Trevor didn't get fired so easily). The scene where Trevor visits Miller in his garage is likely an illusion with a random stranger.

Stevie (the Prostitute)

Completely real. Although, it's more likely that they were in a relationship and Trevor is imagining her as a prostitute to depersonalize it. He is the crazy psychotic ex she is talking about, and he likely gave her the bruises.

Immersed in one personality, he abuses her. In another personality, he loves her and pays her well out of guilt.

Maria (the Waitress)

Mostly fake. The only real scene of her was when Trevor runs her child Nicholas over and she is in grief. The carnival date, nor the romantic night never happened. Trevor, thinking the waitress is Maria, tips her generously out of guilt

Nicolas (the Child)

Mostly fake. The only scene real scene of him was when he got run over by Trevor. The scenes in the carnival, the satanic funhouse and Ivan's kidnapping of the boy were all hallucinations. His hallucinations of rescuing the boy stemmed from his guilt and desire to want to make up for his mistake.