Frankly, it feels like it would have made more sense to say that if they're not going to differentiate by region of birth, then at least, Cubone should evolve into Alolan Marowak at night, but Kantonian Marowak during the day; the night provides the 'mysticism' for its spirituality, and Shiinotic and Mandibuzz are at least believable as nocturnal hunters. Take the opposite route for Exeggcute and Exeggutor. The lore already states that Alolan Exeggutor gain more energy from the access it has to the sun in Alola, so it only makes sense that it wouldn't become as powerful at night.
Then just say that Pikachu evolves into Alolan Raichu when it's Amie happiness is maxed out, like Eevee into Sylveon, or when it's full of beans. If they can merely indicate pancakes as an excuse for a branched evolution, then I think that they can get away with the explanation that these three only need to be in Alola to achieve these forms under these conditions. I'm not sure how they'll handle this in the future as it is without introducing some new item or mechanic, or just locking these forms to Alola exclusively.
Then just say that Pikachu evolves into Alolan Raichu when it's Amie happiness is maxed out, like Eevee into Sylveon, or when it's full of beans. If they can merely indicate pancakes as an excuse for a branched evolution, then I think that they can get away with the explanation that these three only need to be in Alola to achieve these forms under these conditions. I'm not sure how they'll handle this in the future as it is without introducing some new item or mechanic, or just locking these forms to Alola exclusively.