Well, changing the fluid didn't work. More precisely, it worked for about 10 miles: by the time I got to my office the pedal wasn't returning properly again, and on the way back to the shop it stopped letting me change gears. Again.
The ZSpeed HD replacement CSC is on backorder.
I don't have that kind of time/money to spend on the rental car, so the shop is going to call around in the morning and see what they can find. I told them that worst case they should go ahead and put another OEM CSC in, and maybe it'll last another 20,000 miles and give me time to get the ZSpeed HD. The guy will call me tomorrow and let me know my options.
Enterprise gave me a 2016 Hyundai Elantra. I MISS MY CAR.