Yes, it could cause an issue if it is left on for too long. You might turn the knob and depress it and turn again to turn the gas off so that won’t happen. Peace.

The heater is still full of water even though the water is off, and the heater will only light up when hot water is required so there is no danger. I would only recommend that any pilot light is turned off to prevent any evaporation to the water in the heater by the pilot light.

As long as there is water in the water heater, then it is fine. If the water heater was drained, then the heating element would probably burn out and need to be replaced.

The answer is no it will not harm it. And leave the gas on as long as there is still electricity to the home.

If the water heater is drained there could be a problem with it overheating. As long as it is full, no problem.

no, but to save fuel cost, turn knob to off