Hey Todd I'm kinda new and don't check post on a regular basis.I think I can be of assistance if you don't mind going through it again. Basically it seems you've come to an conclusion...Thermostat stuck open.
The NSX has 2 water temp. sending units on the engine.The(2wire) one mounted directly on the thermostat housing cover associates with the fans for the radiator and ecu. mounted just beind is the(1wire)sender which has a YEL/GRN wire attached to it.This sender is only for the temp. gauge and doesn't effect any ECU functions or the cooling system,just the temp.gauge.With that in mind I need to know if your fans ever turn on,and when.When you say "temp gauge going down..."Does the gauge at first warm to normal position the go down as you drive or gauge doesn't get as warm,or gauge doesn't move from cold.Also are you saying your car is cold because the gauge says so or because engine feel cold to the touch.
If fans come, on engine is hot,so I would check (1wire)sender.This is very easy to check....
1)check #5 fuse under-dash fuse box.IF OK go to next step.
2)disconnect YEL/GRN wire from sender and with another piece of wire attach to YEL/GRN wire and ground it.
3)now sit car and turn ignition on, while watching gauge,which should be sweeping to "H" mark.TURN OFF IGNITION BEFORE NEEDLE REACHES "H"
If gauge moves then sending unit probably bad Remember I don't know what was previously discussed,forgive me if your beyond this point.Though even if you had taken the Thermostat out, the motor should still get hot,to me stuck Thermostat isn't the problem.
Well I hope I was able to shed a little light .