"Another plausable scenario which fits with the pilot's story is that it had been running but he shut it down when he noticed the tie-downs. Prop stops not far from TDC and he bumps it just enough to push it over the top."
I don't think that is plausable aerocat. If they did shut down the engine the the mixture an throttle would be pulled back, an if the prop was knocked passed TDC, the plane wouldn't have started an accelerated up the back of the other one (as it looks like it did from the pics). The throttle must have been open quite a bit, because if I remember correctly oenpelli's apron slopes upwards in the direction those planes are facing.