I'm against Zeraora for the reason you pointed out: Kukui said in SM140 he doesn't have a Pokemon that can use an Electric-type move. It would be really stupid for him to suddenly have Zeraora after saying something like that, when the safety of Alola was at stake.
For Marshadow, it's not wishful. Marshadow would destroy Ash if he actually had one...I'd rather not Kukui have such an OP Pokemon.
It's more that the Gen VII era only had two movies, with two Mythicals, and Pikachu happened to battle both in those movies. Prior to this, Pikachu didn't battle the Mythical Pokemon, usually Ash & friends tried to help them out (Victini, Keldeo, Diancie, Volcanion/Magearna) or a legend battled them (Darkrai, Shaymin, Arceus, Genesect, Hoopa).
Then when Zeraora surprisingly debut in the main series and battled Pikachu, I thought it was possible Marshadow has more than a cameo and plays a bigger role in Alola. So far, all of the Gen VII Mythicals have big roles in the cast, except Marshadow.