The difference between you and this person is you're not mentally ill, and the law IMO correctly evaluates a person's mental capacity in many other settings, starting with the presumption that minors cannot be bound by contracts in most circumstances, appointment of guardians, and more.
And I don't really understand the distinctions you make above. If he's completely responsible for his own acts, and society has no business inquiring about his mental capacity, which is what you argue, then why do you support the family's ability to bankrupt her in civil court? Or support her being held criminally responsible if, for example, he decided to kill himself by running into a truck, that also killed the truck driver. Either she is or is not partially responsible. If she is, that's fine, but I don't see how you can carve out this case (no criminal liability) but then assign blame to her if he chose another way to kill himself that also happened to harm others.