If I have a Pokémon with Mold Breaker ability, Cranidos for instance, and I use Head Smash on a Pokémon with Mummy ability, such as Cofagrigus, will the Mummy ability be transferred to Cranidos? Or will it be nullified like other abilities?
When a Pokémon with Mold Breaker is brought out, the message " breaks the mold!" will be displayed. Mold Breaker nullifies the same Abilities as in Generation IV, as well as the following Abilities (even if they would end up boosting the power of the Mold Breaker Pokémon's move): Big Pecks, Contrary, Friend Guard, Heavy Metal, Light Metal, Magic Bounce, Multiscale, Sap Sipper, Telepathy, and Wonder Skin.
No Mummy is listed.
So yeah, it will transfer.
Yes it will transfer Mold Breaker only nullifies abilities that would prevent a move from working. For Example: making it so you can use a ground move on a Pokemon with Levitate.
Yes, this happened to my haxorus. The mummy got passed on to him and it wasn't any different than other situations.