One more idea came to mind here. Get a nut that fits the crank shaft. Slice it with a hack saw, cutting into the threads. Once you have the nut cut, screw it onto a screw to clean the flashing from the cut up, then screw it onto the crank shaft, almost to the prop nut. Now, and this is the only place to use them, really clamp down on the split nut with your vice grips. You want the jaws placed so the cut is at 12:00 and the jaws at 9:00 and 3:00. You should be able to effectly lock the threads. Now try to unscrew the prop nut. When it breaks loose, remove the vice grips and the split nut. Your threads should be in good shape.

One question, did you use thread lock on the nut? If so, this is why thread lock on a prop nut is not a good idea.

Don