I'm confused about something

Well, it's more complicated than that. The thing is: Nick and Amy both pretended to be different people so one would be attracted to another. Nick is a relaxed, egotistical, laid back guy, however, he pretends to be romantic, witty, sharp and game. Amy is controlling, methodical, insecure and paranoid, however, she pretends to be as easy-going and relaxed as Nick. They both have sociopathic and narcissistic tendencies, so playing a character isn't very hard for them when they are living a fairly wealthy life in NY. But when they lose their jobs, become poor and move to a place they consider below their standards, they begin to face real marriage problems. And, suddenly, playing someone they are not doesn't seem really worth doing anymore. The thing is: is not Amy who's awful to Nick, it just happens that they both take off their masks and they find out they hate eachother. They are both equally toxic presences to one another. Their marrige becomes a mere convenience: Amy's with Nick because she's too much of a narcissist to come back to New York divorced and poor, and Nick's with Amy because he depends financially of her.

This is the main blunder:

If the cops knew about Desi, why didn't they contact him?
After all, Amy had a restraining order placed on him.

Due to the sociopathic behavior she had displayed with the other men that we know of in her life we can safely assume that Nick's life with her had been just as he had described to his sister.