Updated my post with specific tests. You've done #6, so we can skip that. Do others in order. There are several independent systems involved here:
1. gauge + temp-sensor - purely for human consumption. Doesn't affect thermostat or fan operation in any way. You can remove gauge+sensor, the thermostat and fan will operated as intended.
2. thermostat - controls physical water-temps via radiator. Doesn't have anything to do with gauge or fan. You can remove gauge+sensor and fan, and thermostat will still operate as designed.
3. fan & thermoswitch - control air-flow through radiator. Doesn't depend on gauge+sensor or thermostat to do its job. Which is just to turn ON fan at certain water-temp and turn OFF fan as water-temp drops. The spread between OFF and ON temps is to prevent rapid ON/OFF cycling of fan at transition temperatures. On many racing-bikes, fan and switch are removed to save weight.
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