One symptom down, one to go. The squeak is fixed.
Asked for advice and for help on the MSCCNC forum, and one of the guys volunteered to show me how to take the accessory belts off the car, so I could test the theory that it was indeed a belt squeak (see Part 1 for details).
He showed me all the bolts that hold the two belts in place, then he took off the belt that drives the air conditioner and power steering, and then I put that belt back on. Of course it was not that belt. But the other one, driving the water pump and alternator. One of the pulley bolts was loose, and after it and the tensioner were re-tightened, the squeak was gone.
To get to the first belt, we had to disconnect the air temperature sensor on the intake, and of course, even though the car was not running, the computer noticed that and registered this as an error code. Oh well, big deal, I thought, but stopped by a tire/alignment shop on the way home, and turns out, another error code was there--the cylinder misfire.
My attending mechanic was not happy that I had the misfire code cleared the last time, so he had nothing to go on, now he's got the code, and hopefully it's nothing serious.