With normal running the engine tries to tighten the prop nut everytime it fires so a left hand thread nut would be counter productive. At all times other than on the power stroke the prop is actually driving the engine, which means of course that the prop and engine are slowing down slightly. A backfire when starting is the most likely to throw the prop because the starter is applying a torque to the prop nut trying to unscrew it plus with the back fire the engine is trying to go backwards.

A backfire can only happen if the mixture ignites too soon or the pressure rise after ignition is too rapid. Too much power from the glow driver makes the plug glow too hot and advances the ignition on start up. The ideal is to have the plug glow just enough for easy starts and no hotter. A lean mixture burns faster than a rich mixture so even if both begin to burn at the same point then the lean mixture will build up pressure much faster which can then drive the piston back down before it's had a chance to go over TDC. Another culprit is too much nitro which has the affect of advancing the ignition point.