It's good to see you back PinkFlamingo.
I'm so sorry things have been rough. What your boss did really pisses me off. Mental Health is not separate from Physical Health, and I've never heard of a difference when missing work as you were "incacipated" for work.
Just because you signed that form doesn't mean you can't take any other action. It sounds like you were under duress at the time. They wouldn't even let you leave the building. The are really wrong. I would try to pursue your severance pay. If appeals to you, call NAMI. Fight for you, take the anger out on them, not you.
Hugs.