I'm wondering if this woman really IS a nutcase; with a documented illness such as bipolar or schizophrenia. In which case she may be using the ADA against the drs. in the office. We had this happen at a company I worked at. The woman did all kinds of the same erratic things that Sue is doing. She didn't show up, called in sick, disappeared for two hours as lunch, showed up acting drunk, did her work wrong, got in arguments (not just with other employees but with clients!), lost us two clients, etc. Well, despite multiple write-ups, requests from other employees they could do NOTHING to her. She had the ADA on her side. We had a company meeting to about this one employee which technically they weren't supposed to do but they had soooo many complaints from other employees they had to justify their actions or lack thereof. According to one of the owners, who was a lawyer in a previous life, they couldn't fire her because according to her all of her behavior was the result of her disability and she could sue and win if they fired her. The woman was written up time and again but until she quit their hands were tied. Sounds like your office may be in the same situation.
I'm sorry you felt compelled to quit and hope that in the end you find an even better job.