It's hard to tell. There is more lore in Kalos that would've supported it, but none of the mythicals of gen 7 have context in Alola. Only Marshadow could fit at least aesthetically. It makes no difference, Magearna is gen 7 and that's that.
As for the palette swap, that I am certain they intended as a movie tie in, but abandoned it for some reason. Their only chance to do it has long since passed, there is no reason to do it now. Yes, like Light of Ruin, it will remain a dummied out idea.
