Sounds like you have multiple problems interacting at the same time.
This is what I recommend you check.
Do an Ohm reading on the gauge temp sensor as well as the ECU sensor. Specs are in the manual. If the Ohm sweeps check out then it is likely that the engine temp is really low and the ECU is stuck in cold start mode. Is it possible that there is no thermostat installed? or is it stuck open? easy check: when the car is dead cold, pull the radiator cap and start the car. Once started, rev the engine and observe if the coolant is quickly moving in the radiator. If so, at a cold start the thermostat should be closed, and NOT allowing coolant to circulate quickly.
I would try to solve temp issue first, then work on idle.
Hope this helps,
FRO