Maisie's quote should be taken with a grain of salt at this point, maybe she was told to say so to not spoil the whole Stoneheart thing. They could "splice her in" similar like they did with Theon's arc. On the other hand, as much as I'd like it, I doubt they show her ressurection in "Mhysa", as it would require Dondarian and his guys appear out of nowhere. They could have been following Arya and the hound the whole time, though. Can't remember how the timing's panned out in the book.