Made the test indicated by Arthur last night.

Key off.
Pulled connector from 2-pole sensor on thermostat housing.
Resistance between wires: 29.6K ohms
Resistance between brown wire and ground: basically none (30 or less ohms)
Resistance between red wire and ground: 29.6K ohms
Resistance between poles on sensor - 486ohms
Turn key on - temp gauge in cluster indicated about 85C at this point.
No "early turn on" mod installed on this car.

Pulled the sheath on this harness leg back as far as I could (3 or 4 inches) - insulation, while a bit stiff, is resistant to any attempt to break or fracture under thumb-nail pressure.

I replaced the controller in the car with one that was original to the car that I'd had rebuilt by Programa. Pulled error codes this AM after my drive into work:
E040 - ECT Sensor - Short Circuit (continuous fault)
E041 - ECT Sensor - Short Circuit (intermittant fault)
E043 - ECT Sensor - Open or Short Circuit (intermittant fault)
E075 - Coolant Circulation Pump - open or short circuit (intermittant fault)

E013 is gone, as expected, because this controller is for a car without rear A/C.
E073 is gone, but that was an intermittant fault - it might or might not show up again.

Sensor values still show HI for the ECT sensor on the new controller.

I am 95% positive that the previous controller, regardless of where I set the dials, would not ever activate the A/C compressor.

This new controller will allow A/C. Setting the dials to LO nets cold air out of the ducts in short order.

What do we know now, based on the previous tests?

1) The controller is likely not the cause of the erroneous readings. Two different controllers, one recently rebuilt, report same (or close to same) trouble codes, and comparable sensor values.
2) The sensor itself is not the problem, as the 486ohm is within the acceptable range for 85C coolant (460 - 650ohms, per AllData).
3) Wiring values are way out of spec. (Should have seen 1.3k, got 29.6k) I'm pretty sure there should be little to no resistance to ground for the brown. The values I got were close to 0 resistance (30 or less ohms). That wire is the ground reference wire (according to AllData's diagrams) for all of the sensors in the HVAC system, so I'd think if there was an issue with that wire, possibly I'd be seeing all sorts of other problems.

One thing I have noticed is that the fans come on on Stage 1 when you first start the car. As the car warms up, they run faster, but I don't think we get to Stage 3.

What is curious to me is that the specified resistance values for the sensor are 5-8k for a cold engine (20c), and 190-220ohms for an overheating engine (120c). However, the circuit resistance is baseline at 29.6k, which confuses me as to why the controller thinks the car is overheating. I guess I just don't understand what the controller expects to see in this circuit...

I'm thinking I need to check the wiring harness further...

What say ye?