"But Pokemon like Alolan Raichu and Alolan Exeggutor will probably say they evolve from plain ol’ Pikachu and Exeggcute since there’s no Alolan Pikachu or Alolan Exeggcute (as far as we know)."
I keep saying this and no one seems to see the logic in what I'm saying. It'd make no sense for a normal Pikachu or normal Exeggcute to evolve into Alolan Raichu and Exeggutor because it would mean that you couldn't get normal Raichu or Exeggutor without transferring as both Pokemon evolve with stones. So unless a new stone is made for both of them, they only evolve into their Alolan forms in specific parts of Alola or, as I keep saying, they have Alolan Pikachu and Exeggcute which they haven't revealed yet, normal Raichu and Exeggutor will be impossible to get in-game without transferring.
The existence of Alolan Pikachu and Alolan Exeggcute makes sense when Alolan Cubone and Alolan Persian are practically confirmed (a ground type evolving into a ghost/fire type would have serious problems and it'd be ridiculous for an Alolan Meowth to evolve in normal Persian) and the other Alolan Pokemon are all full evolution lines (Sandshrew/Sandslash, Vulpix/Ninetales and Rattata/Raticate).
(I apologize for my rant)