Seems to me it's worth considering that you don't rely on medication, but also talk with professionals about how to deal with stress (and that susceptibility to stress can be caused by other things ... things that may be causing your anxiety disorder).
Not sure why they'd have to also make the graphic design person supervisor of a different dept. (except to justify an increase in money that they didn't have to give him in the first place regardless of how many jobs/duties he was assigned).
I wouldn't presume that it is your disability that has caused this change. It may be that he didn't want to supervise anyone, let alone another dept. He may be having marriage or personal life troubles. Sounds like he has severe "issues" that have nothing to do with your disability, but rather you're a handy/convenient target.
If your medical cert says that you may need to use time off due to a manifestation of the serious health condition (whether it was specifically named or not), and that this condition isn't something that will go away within the next six months, then it is what it is. If your GM knows why you left, then they aren't entitled to demand you come into work. You're free to file a complaint with the federal DOL.
You're free to contact the police if you fear for your safety, and put the company on notice that you do.
"I can't do this anymore."
If that's true, then you quit and file for unemployment benefits stating that you believe it's a constructive discharge situation because the employer won't do anything about this person's behavior. Your health is more important than a job.

