> It's a minor difference though as there would be no way to detect thata single grounding screw had failed and then the failure of the second screw would cause the same problem.

Actually, having two grounding screws would have avoided this problem - the flap switch would never have been connected to the ground via the feathering mechanism (unless both screws got loose and somehow entangled). In that case, either the flap mechanism would have failed, which would have been obvious, or the second terminal of the feathering switch would cease to be grounded, which could lead to sudden feathering with sufficient EMI. While still not great, a dual ground-screw setup would have been much better.