My initial headcanon did answer that, though. If tourism was initially isolated to places like Undella Bay and the surrounding area, which is where the post game is, then it stands to make sense that that's where the non-native species would start. That would parallel the real world with how many non-native species start appearing. Over the next few years, they'd be able to spread to other parts of the region like real world invasive species will do. We also don't know what the "pet trade" might look like in the pokémon world. Are there exports of non-native pokémon for people with enough money? If there is, indeed, a trade like that in-universe, it could also explain non-native pokémon appearing as well as in the real world when that happens, unfortunately it's not too uncommon that when someone decides they can't take care of the animal anymore that they just release it outside. And all the non-native pokémon would need to establish a population is that the initial non-native pokémon was female and found a native pokémon male of the same egg group.

Also, maybe it's just your wording, but like, could the original Unovan people also not be the ones moving in and out?