I agree that one bolt is a bad idea.
Maybe try tapping threads into the trans case and use a shorter, but thicker bolt. Maybe 12mm or 7/16 threads? While the bolt should go into the engine block, I think it'll be OK if it doesn't. You'd probably have to enlarge the hole in the starter for the larger bolt.
Or have the broken bolt side of the starter welded to the trans case.
Or always park on a hill!
