Welllll, my experiences and training in such matters in San Diego is 26-30 or so years old.
I did witness the 'freeing,' 'liberating' of the mental hospitals of the 'harmless' mentally ill such that they began populating the streets as homeless.
And, I was certainly trained to use such criteria as you mentioned for forcable incarcerations.
But in my experience, back then, the criteria for voluntary hospitalization was NOT THAT strict on all those criteria--certainly not at all hospitals.
Shoot, I know one Navy Commander (CDR, USN) who was hospitalized in Balboa for--I forget--2-3 weeks or so--at least 10 days--and ran through an extensive battery of tests. His problem? His shipmates found him in the parking lot on his knees with his arms raised worshipping because he saw a huge cross in the sky.
They later released him as totally sane and OK but advised him that the next time he saw the cross--to avoid kneeling down and worshipping in Balboa's parking lot. He could routinely still see the cross and had a profoundly deep spiritual awakening that changed his life for the better.
I had one patient at a Baptist counseling center--a real controlling wife and mother who was hyper manipulative, devious, blaming etc. who had been hospitalized several times. She was not usually suicidal--though she could play that game with great histrionics to get her way. And she was not homicidal nor really battering.
I don't recall what the diagnoses were--more than one DSM III category, as I recall--but her husband told me hospitals in Southern California REFUSED TO TAKE HER ANY LONGER--not because they thought she was particularly sane. And, not because they thought she would avoid being a danger to herself or others on occasion (though that was NOT a consistent problem); but because they could no longer tolerate her troublesome--no progress, outrageously manipulative, obnoxious personality. She WAS a REAL DOOZY.
Course there was the very wealthy 20 something year old son in Taiwan--who WAS a danger to others--at least when he decided on the top of one of his dad's 13 story buildings to start pushing marble furniture off the building onto the sidewalk below. As I recall, the police weren't nearby at the time and the dad wouldn't testify against his son so he was not hospitalized at all.
I realize that the professional could impute possible/probable harm based on an interview or other convincing criteria. But some people were voluntarily hospitalized for chronic depression when there was no probable suicide risk assessed as valid.
I don't really care what your training is/was or the law etc. I experienced what I experienced; observed what I observed. Perhaps times have changed, tightened up on the criteria in our litigious society.
As I recall, another criteria that could relatively easily facilitate a voluntary hospitalization was any seriously convincing evidence of psychosis--even without any signs of it being dangerous to self or others.
BTW, the woman I mentioned who's husband told me the hospitals would no longer take her--had been hosptiatlized 8 times--sometimes for months at a time. And, as I recall, my boss at the center checked the facts out and just shook his head about the case.