I tried the resistor trick - no effect. I didn't have 500 ohms, only 468, but according to AllDataDIY, that should correspond to about 85C.

I didn't reset the fault codes in the controller before hand, if that matters.

Did check them and here's what I got:
E013 - no connection to rear A/C control
E040 - ECT Sensor - Short Circuit (continuous fault)
E041 - ECT Sensor - Short Circuit (intermittant fault)
E073 - Heater Supply unit coolant circulation pump (intermittant fault)
E075 - Coolant Circulation Pump - open or short circuit (intermittant fault)

E013 can be ignored - the control unit is from a '92 600SEL, which has standard rear A/C. This car doesn't have it.
E073 & E075 have been persistent problems with the car, even after I replaced the Duovalve pump assembly. As I understand it, these errors are a system issue and routinely show up, even when there's no "real" problem.

E040 and E041 point to the ECT sensor, which when bypassed with a 500ohm resistor had no effect. However, again, I don't know if I needed to erase the fault codes first.

I've erased the fault codes and will check behavior again with the resistor.

The diagnosis tree is fairly simple here - it's either 1) The sensor, 2) The wiring, or 3) the N22 controller.

I'm pretty sure I've replaced the sensor, but the resistance values are easy to check per AllData's values.

The wiring was moved when the headgasket job was done, and I've not checked the integrity of the wires in that sheath - that is something I need to do. It is definitely possible that the insulation on that leg of the harness is damaged - it lives in close proximity to the hot motor. (I didn't check that leg of the harness the other day as I didn't yet know it was the sensor values causing the problem.)

I'll do a little more digging and report back my results.

Thanks to all that have chimed in!