Dave,
Your idea on the redundant electric motor might work but could you also get all the props turning in synch with each other like a mechanical drivetrain would? I would suspect that having one or more get out of phase might cause some kind of vibration issues. I'm sure it could probably be done but how much complexity would it add? I guess if the props are balanced individually they shouldn't care what the other props are doing but I suspect that aerodynamically the interaction between props and airframe and prop to prop might cause some issues. Just a thought.
Max