Wow! What a jerk your boss is! I think there are a few things going on here:
1. Since you had just be hired on permanently and the dispute with the keys was with somebody who'd been there for a while, especially your boss's own personal assistant, your boss took her assistant's side.
2. Some people just do not believe that psychological disabilities like panic attacks are real and don't take them seriously. She sounds like she is one of those people.
3. This really sounds like a case of "misery loves company". Many people feel like work always has to come first and are scared to take any time off even when they are legitimately sick. When somebody else takes a sick day, they are quick to judge. They expect everybody to just shut up and bear it and put the company first. There is nothing wrong with balancing your job and your health, but in too many industries and for too many individuals the job always has to come first and asking for any type of concessions for your health is viewed negatively. Your boss is probably one of those people with that attitude.
I think she has already decided that she wants you gone. She is hoping to replace with you who will go along with anything she says. Of course, she knows the law is on your side and if she just fired you, you could sue the company. She is hoping that by asking you to provide more and more doctor information, you will either quit on your own or not be able to provide the necessary information and then she can use that in court against you.
Have you spoken to HR about what is happening? It's possible they may be able to get her to back off if they hear your side of the story.
If it continues after you've spoken to HR, I would look into speaking to a lawyer about your options. Their might be local civil rights groups that can help you out or tell you where to find a good lawyer. If not, you can check your phone book for lawyers that specialize in this type of a thing. Some times they will offer a free first consultation or hold off on having you pay anything unless you sue and win.
I would also hold off on going to the doctor that your boss wants you to see until you speak to a lawyer about whether or not it's a good idea. I'd be nervous that she'd find some quack doctor who will give you the diagnosis the company wants to hear and then use that against you if it gets to court. You already have two doctors who have testified to your disorder so that should be enough.