Laws re Insanity and Commitment

I've been watching the news. I'm confused about the laws concerning commitment of someone who is considered "insane."

People can be committed against their will if they pose an "imminent" threat. What does imminent mean? Do such people have to be walking around with a weapon? Actually threaten somone?

People can be kept in an institution for up to 72 hours--against their will. Then what?

Someone is sent for a psychiatric evaluation and is deemed a threat to himself or others. This person remains in the institution and is treated with drugs. The medication appears to be working. The person is released with the understanding that he will continue taking his meds. He doesn't. Does anyone know? Can anything be done about it? Is there any kind of follow up?

Someone is sent for a psychiatric evaluation and is deemed a threat to himself or others. This person remains in the institution and is treated with drugs. The drugs don't appear to be working. How long can the person be kept in the institution? Indefinitely?

If someone is suffering from a serious mental illness, how can this person make a "rational" decision concerning institutionalization? In essense, how does one define "free will" or just plain "will" for such a person. (Can't be held against their will) I'm not talking about someone who is a threat necessarily. Just someone who has some kind of mental disease.

Someone is sent for a mental evaluation. This person is deemed to have a mental illness but is not a danger to himself. Can this person be institutionalized regardless?

I don't mean to get too technical, but the laws regarding people who are not rational seem just a bit meshuga to me.