So, how does one determine which mentally ill people are okay to own/carry a firearm and who can't?
I think an easy definition is the probability to commit harm to yourself or others. medical authorities use that definition to involuntarily hold folks for 3 days when requested by a family member.
The potential for abuse is obvious in that statement. I worked in inpatient "crisis stabilization" psych for 5 years, the amount of abuse is staggering. Say there's a child custody battle, "my ex said she was going to kill herself". Put in for observation, nice little tidbit to tell the family judge at the hearing, doesn't matter that her ex was lying. It doesn't necessarily take the full 72 hours, but you'll wait until a doc shows up to eyeball you.
I absolutely agree that people with certain mental illnesses and behaviors should be prevented from owning weapons, but the standard of proof needs to be very specific with regards to checking that particular box. THEN, in the interest of maintaining confidentiality, that checked box absolutely needs to be nothing more than a checked box, just "denied".