No one but you can decide. The whole reason for a water bed heater is to make the people sleeping on the bed comfortable.
For that reason, the temperature should be raised or lowered according to your comfort.
Lower than body temperature
No, you will have different temperatures of water inside the water heater, which will make it difficult to get a consistent temperature out of a shower valve or faucet. The heater will also not work correctly or efficiently.
Should be on the lower rear of the block, near the dipstick.
Hypothermia
You don't. If the heater has an electronic spark, then it doesn't have a standing pilot to light. The spark lights the pilot when there is a call for heat, which in turn lights the burners. If the heater is on and the pump is RUNNING, the heater should light automatically when the water temperature is lower than the heat setting.
Stay at the factory temp.
The Temperature should be at least 40 degree Fahrenheit of higher for it to dissolve any temperature lower will take a long time to react or it will not react at all. The higher the temperature, the faster the dissolving and reacting.
The air from the vent should be 18 to 22 degrees F lower than the temperature of the air going into the return.
Place the container of water in a vacuum & it should start to boil at room temperature.
In any vehicle, when the radiator system is working correctly, the temperature gauge needle should be just below the half way mark when up to operating speed and temperature.
At least 20 degrees lower than the air temperature.
yes absolutely