Snyper, I stand by my assertion that PTSD, in itself, is a clinical diagnosis, and not an adjudication of someone as being "mentally defective."
Now, can a person with PTSD be further declared to be "mentally-defective" due to effects or other attributes of the disorder? Certainly, but then we're dealing with that declaration, and not with the initial diagnosis of PTSD.
Earlier, you made the snide comment of:
I doubt there is a category for "disabled but otherwise mentally stable"
Why should there be? The section of code to which you were referring identifies
prohibited persons, not those
excepted from those prohibitions. A person with a disability, but who is not identified as a member of one of the two prohibited classes of which we are discussing, is not among those prohibited. The law stipulates who is prohibited, not who is not.
In the scenario described by Bartholomew Roberts above, the two doctors agreeing to hold a person for further observation (assuming it's against the will of that person) meets the "involuntary institutionalized" criteria, perhaps even before any diagnosis is made, or any declaration of incompetence/defectiveness is made.