All right, still working on this. While we had the machine open to look at the thermistor, we (re)checked all the drain hoses, the filter, and even the closest thing to a vent that we could see in the machine (it's labeled as part M310 in the service manual), and cleared out any accumulated lint, etc., just in case that fixed the problem.
We then tried a test wash and dry cycle, but when we checked the machine at a time that should have been part of the way through the full wash and dry cycle, the washer already had the "finished washing, press any button to open door" message. The tub had water in it, and the door wouldn't unlock.
We took off the front panel again, unlocked the door, drained the tub manually, checked the hoses for blockage, and tried running a speed wash cycle, without the dry cycle. The tub didn't spin, and the machine eventually gave a "check drain hoses and pump" message. So we rechecked the drain hoses and looked at the pump, drained it manually, and ran a speed wash without dry cycle again to check if it would drain properly ... which it did.
So we tried running another full wash cycle, with drying. The washing part of the cycle was going fine when we left the house to run some errands. When we came back, the machine had the "finished washing, press any button" message, but the tub had water in it again and again the door wouldn't unlock.
Does this still point to the dry thermistor being broken, or does it sound like there's a different issue now, or an additional issue?
Thanks for your help, as always!