I tested most of these questions. To answer in order: Yes, no, didn't test, Only on primary (Assuming same with skill swap) Trace and mummy only work on primary ability, no, no.
https://replay.pokemonshowdown.com/trashchannel-gen8multibility-3146-8jax3r5whpqykbikhvpzxsu0xm1ykjtpw
I'm gonna take...