Got the plugs on the coolant bridge the wrong way round, and stuffed them? as one's 5v, the other 12v... plug the wrong one and it fries the smaller sensor.
Not sure this would cause these issues, but can you rule this theory out, for peace of mind?
Doesn't apply to the gauge sender
The gauge, ecu, thermostat and fans are all completely independent unless the ecu sees a dangerous temperature and it can then over ride the radiator fans.
Sounds like there's more than one issue here as the thermostat should be doing the basic coolant flow throw the rads and the fans doing the fine tuning
Aircon working as it should? If the answer to this is no, then all bets are off because you instantly loose two thirds of the fans. It is possible that the thermos switches are the wrong way round on the radiator although iirc the wiring position doesn't allow you to plug them in if you swap the upper and lower ones
could be wrong...