You're all awfully helpful. And this situation keeps getting worse and worse. I had the car brought back to the independent mechanic (not a franchise) who had first worked on it so they could fix any of the damage they had done.
They had insisted the front rotors weren't too thin, but replaced the rear rotors and calipers. I paid for the rotors. I also purchased an original Mercedes master cylinder and sent it to them to install; the master had broken when my calipers went.
I went in to pick my car up yesterday (and the frightening things I saw there, I can't even begin to tell you).
Now my brakes are usable, but I'm out $700, and I should still replace the front rotors and calipers.
New problem: the car no longer shuts off after removing the key. I know it's a vacuum leak, but could it have been caused by this lousy brake job?