Question: Why does the Chief fake being deaf for much of the film?
Question: I'm not from the USA so excuse the lack of knowledge, would pleading insanity really get you off a rape charge?
Answer: 1) McMurphy didn't exactly "get off" by reason of insanity; he was still incarcerated for an indefinite amount of time, just in a psychiatric facility rather than a traditional prison. 2) He was originally sent to a normal prison for the statutory rape charge, but is then transferred to the mental hospital due to repeated acts of aggression that suggested some form of psychosis (or, as the doctor suspects, faking it to get out of hard labor). 3) No, it wouldn't. The insanity defense is a) very rare and b) very hard to prove, and it would be difficult to apply to rape, statutory or otherwise.
Answer: Insanity, legally, is defined as not knowing right from wrong. It can also be "temporary." It can only be diagnosed by a licensed psychiatrist, and it is rarely determined as such. Laws vary from state to state, but if a person was guilty of a serious crime and was found to be insane, they'd be confined to a mental hospital, either long-term, permanently, or, if they sufficiently improve, they'd be either be released after a certain amount of time or transferred to a general prison to complete their sentence or remain there indefinitely.
Question: I don't understand the scene where Candy and Billy go into a room to have sex. McMurphy tells Chief that they will leave for Canada when Candy and Billy are done. Why did they stay there all night and fall asleep? Why did Candy stay in the room with Billy?
Answer: The girls who sneaked into the ward had brought alcohol and everyone was partying before McMurphy and Chief were to escape. Everyone got drunk while waiting for Candy and Billy to finish, eventually passing out. Candy and Billy probably just assumed McMurphy and Chief had already left or they'd fallen asleep.
Answer: It's better explained in the novel, where the Chief is the narrator. Essentially, he played deaf and dumb, so he'd be left alone by the Nurse and the staff; seeing what they did to the other patients, he figured the less he was noticed, the better. In this way, he is privy to a lot of the seedier goings-on, since they don't think he can hear what they're saying or tell anyone what he's seen.