The domestic violence decision would have to be on the cops on the scene IMHO. If they arrested the person, then they must've thought the persons was dangerous enough to take into custody to prevent future violence. If so, I think that right there is good enough reason to confiscate any guns. And if not, well then they leave the person there without confiscating the guns.
If the person's mental state is in doubt, I would think the cops are not up to making that call. If they take the person to a mental ward or facility then a mental health expert would have to make the call over any weapons confiscation. And obviously any confiscation ought to be reviewed after 14 days or whatever the period of time is before the person can make his/her case to get his/her weapons back.
Do you not think we ought to try to keep guns out of the hands of those who would do harm to themselves or others? Or do we just let shit happen?