Jan 2020 update.

Got access to the car again over holidays. It had popped a CEL a couple of weeks earlier that was still on when I started the following:

1. found a loose vacuum connection on intake m'fold. corrected.
2. Swapped in new diagnostic module under driver-side dash. On-board blink button now works, as does query through socket 3. So the old module must have been internally shorted.
3. Swapped in new MAF sensor.
4. Checked for tight brake caliber/ wheel bearings. All normal.

If I'd had time I'd have done this sequentially to get more information but I only had a couple of hours with the car. I still don't know where the second coolant sensor is, or if there is one, feeding ECU.

Result: CEL is gone and not returned. Socket 3 shows no codes recorded. Recent mileage check is 18 miles per US gallon or 13.07 litres per 100km. I'm happy with this.