I don't see the sadly in that last setup.
Some #6 in the wall, spliced (in a box and in the panel) to some #12, protected by a _20_ A circuit breaker, going to a single duplex receptacle. Hell, you could change the 50A double pole breaker to a 15A double pole breaker, and wire the duplex as a multiwire unit. The #6 wire would be wasted in this application, but it would remain exactly where it would be needed should the home-owner ever want to go back to an electric stove. I don't believe that the oversize wires themselves would present a problem; presumably the original stove receptacle box was properly sized for the #6 conductors....
-Jon
