My goodness, what a nightmare!!
I'm not sure if I can answer any of your questions, but from my own experience here in CO, if you appear to be a danger to yourself, they (whoever they may be) have to take you to an ER, where a mental health doctor does an assessment and then decides if you should be admitted to a psych hospital or sent home. That doctor is the only one who can put an M1 hold on you. Again, that is my own experience (of which I've had many), I do not know exactly what the law is.
I've once had a similar experience, twice actually. The first time, I had taken Ativan for the first time, and I had some weird reaction to it. I don't remember much, but apparently I called my therapist, and she got worried and called 911. I remember the ambulance people being there, I was in a pretty good mood, not suicidal or anything, but they said since I had (old) cuts on my arm they were taking me to the hospital. I was just like "okee dokee". Somehow I ended up at a psych hospital. I woke up the next morning perfectly fine, they kept me that day just to make sure I was really fine, and then sent me home.

The other time was way worse. It's complicated, so I'll try to make it short. I was suicidal and took myself to the ER. My kids were safely taken care of for three days. But I didn't know what to do with them after the three days, in case I was still in the hospital. I was so stressed about this the entire three days I was there, I couldn't really focus on my issues and getting better. So after the 72 hours were up, they said "Oh, you're not making enough progress" and they sent me to the State Hospital!!! That was a nightmare worthy of some PTSD. The doctor there had no clue what I was doing there either. He talked to me for a good two hours, and then simply said "Well, either they sent me a perfecly sane person and are going HA-HA now, or you are really good". Unfortunately, it was late that day, he was off the next day, and then was the weekend, so I ended up having to stay for almost a full week. The other patients thought I was a staff member, and the nurses clearly enjoyed talking to me.

I don't know what some people are thinking when they stick you in the hospital. It's like really taking the effort to talk to you to find out what's really going on is just too much. It's easier to just go ahead and lock you up, and make you somebody else's problem. (I'm using a general "you" here.)

I'm so sorry you had to go through that. And I'm sorry for my rambling here. Your story just really hit a nerve.
I hope you can get some answers, and I hope you can leave all this behind you.