It has occurred to me that I might have been too quick to damn the HR people at my company. I posted a question about it in a forum where people talk about employment stuff during treatment. Two people have answered - one works in HR, even - and both have said, essentially, "chill, this is what companies do, they don't want to get caught having forced a truly sick person to work so they treat any type of cancer diagnosis like this. It's not you." The person who works in HR also mentioned that my medical expenses probably did not cause the increase in insurance premiums for the whole company.
Sure, the HR woman lacks a lot of communication skills, and seems unable to retrieve a sheet of paper from a fax machine, and seemed to pull that "you can't work until we get that note" rule out of her ass, but those are just unfortunate things. Maybe they are not indications of a plan to fire me or get me to quit. Maybe I've been wrong about this, and maybe I seem like a lunatic. I hope my hysterics can be forgiven, or at least overlooked.