It is pretty simple, and I'm not sure if this has been mentioned before, but here goes...
My main question is, does Mummy get rid of you normal ability and replace it with Mummy, or does Mummy become a second ability like Air Balloon does?
Yes that strategy works it was come up by someone months ago I think it was trachy or maybe DT I am not sure. Anyway this works but your opponent would have to hit you with a physical contact move. I think this strategy is better preformed in double battle where you can just have Slaking hit Cofargius(be sure to max out def and Hp to take minimal damage).