Rarestia - I totally agree. It is very dangerous indeed to label folks ‘mentally ill.’ Who determines that? Under what circumstances? By what set of rules? Someone might call a Christian, for instance ..., mentally ill because he believes sodomy is wrong. Or is not Politically Correct. Who determines? Whoever is on first.
During Obama’s Abuse of Power, for instance, he could have put out an EO determining that anyone who thought Islam was evil was mentally ill. Look at the psychiatrists - they hold a stealth meeting and determine that homosexuality is no longer aberrant.
Who decides? Who makes the rules? Boy oh boy, I want nothing in the government having anything to do with that.
You raise interesting objections, but I still contend that there is a subset of society who should not have a firearm, and likewise should not have freedom. (If the person is such a danger to others that he cannot go around armed, then the same person must be confined because he/she will find a way to harm others given the range of weapons available.) For example, they may convince their father to give them their weapons back.
However as you point out, the government can and doubtless will use this power to confine people with whom they disagree — reference a scene from old Soviet Russia where they supposedly committed political opponents to mental institutions to keep them from reaching a supportive audience. So to implement a scheme where some people lose their natural rights to own a weapon, there has to be a judicial hearing that is fair and representative. Does this work? I for one would like a system that brings back our institutions and uses a just method to determine who needs to be in them,.