Psychiatry has a history of being abused for social control, mostly by psychiatrists. Freud defended rapists, often by using physical violence and long term incarceration against their victims. Hans Asperger mass-executed children at "Am Spiegelgrund". His signature is on the paperwork for locking children into a ward for execution. Jo Erik Brøyn, Norways "top child psychiatrist" (chairman of the council that decides to place children) ... was convicted last year to 22 years for child porn. He had placed 2 children ripped from their parents ... into his own care.
It is not a temporary thing. Psychiatry served to violently abuse children when it was originally created. In fact you can make a decent argument that's WHY it was created. And last year VERY important psychiatrists were convicted for doing ... the same ... also to tens of thousands of children, just in Norway alone.
But at the other end of the spectrum you have paranoid schizophrenia who for obvious reasons don't trust doctors. The CRPD seems to take the view that treating, even temporarily, is illegal and so you couldn't treat the patient to get them lucid enough to consent.
Imagine a case where someone has been unable to get there meds (through some disaster or whatever), and so relapses and becomes paranoid and won't allow treatment. Even if when not relapsed they do want to take the medication. The CRPD appears to say that the patient can never be treated so any relapse must in effect be permanent and their life is functionally over.
It also prohibits incarceration: so you're only option is the American system of waiting for someone with mental illness to commit a crime and then putting them in jail - they CRPD apparently explicitly prohibits putting them in a mental health facility.
It is hard to see how that interpretation is possibly good for anyone - the patient, the patient's family, victims of any crimes that they commit, etc. You could extend the argument to say someone with diminished mental capacity should not be able to claim that in their defense because by definition that would be discriminatory. It's also hard to see how an untreated patient could possibly provide a meaningful defence of themselves if put on trial.
Again, the rationale for the CRPD is sensible: it's trying to prevent things like how LGBT people have been "treated" by physicians, or things like the holocaust which also targeted people with mental health issues.
But the apparent extremism seems hugely problematic.