I metnion the sensor in the bottom of the rad as it it may be where the coolant is leaking from (I'm hoping anyway)Are you assuming the temperature sensor you mentioned in the radiator is the one that controls the temperature gauge in your dash ?
I have a feeling that the sensor in the radiator is for the ECU so it knows the temperature of the coolant leaving the engine or maybe it's the temperature of the coolant entering the engine.
There is another temperature sensor that controls the gauge in your dash. On my 2005 1.9 that sensor is behind & below the tandem pump on the L/H end of the head.
The odd temp readings are probably the thermostat, which I am hoping to replace this week.
the coolant loss is sadly a separate issue :/