well, i was able to get some runtime in today and the pinion stayed on after using blue loctite, but there were other problems...
the teeth wore even more and a tooth even broke off. i was running in fairly slick grass when it happened and the mesh was fine. i checked every other possible cause and everything was fine except for the gear. the motor mount and motor had no movement or play and everything else was fine. maybe the pinion gear was over-hardened. i sent mike a pm with the picture. hopefully, that will be enough to get a replacement without sending it in.