Picked the car up yesterday. Diagnosis is, simply a "low battery". Seems the 5+ hours we spent playing with everything with the ignition in the "run" position without the motor running, with only 8 miles on the car, was enough to drop the battery to a point where the electronics weren't happy.
Service tech told me these cars are extremely sensitive to low voltage, and that they charged the battery, cleared the codes, drove it, kept it overnight, drove it again the next morning and checked for codes again, and all appears to be normal!
Pheww........looks like I dodged a bullet.