Other mistake: The movie is a time piece set in the year 1963. After McMurphy jumps the fence, there is an orange/red Ford Pinto in the scene. Ford Pinto's weren't introduced until 1970. (00:51:27)
Other mistake: In the scene where the Chief smothers McMurphy, when the Chief pulls the pillow away from McMurphy's face, you can see McMurphy's tongue moving up and down even though he is supposed to be dead. (02:03:45)
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.