1 - So since when has an anime only location been prevented from being a part of a region if it has no in-game information? I don't know what anime you watch but in the Pokemon anime i watch it includes hundreds of anime only locations all in Kanto, Johto, Hoenn, Sinnoh and Unova. Nimbasa Town is a notable one.
2 - The name doesn't make a blind difference. If the Dekorola Islands are Islands, why the hell can't they be called Islands?
3 - Referring back to my first point, in-game information has absolutely no effect on the anime. Remember Battle Frontier of Hoenn? Yeah, good call being in Kanto wasn't it.
You were asking if I consider those to be apart of the regions and I said yes because they are documented as part of the regions. Again this new arc isn't set on a tiny island of the coast of Unova. It seems to be set in a fairly large space so to make it part of Unova when nothing in the past would suggest that this new arc would be in Unova would just be weird.
There is a difference between those 1 episode cities(for the most part) and a brand new arc. The reason those cities are included are to extend the length of time they are in a particular region while still sticking to the general game plot.
This has nothing to do with the games so making it part of Unova when there is nothing left in Unova to advertise expect little things here and there would be pointless.
The anime is to advertise the games until they advertised most of that particular region. Then they can take creative liberty and essentially go where they want(orange islands, battle frontier)
The battle frontier is different see. At that time fire red and leaf green were coming out so to make them part of Kanto made sense as to advertise both the battle frontier and Kanto at the same time was smart.
They are done with Unova now so them going to another location till the new gen is also smart as they can explore new places. There is a reason they are going to a whole new place instead of staying in unova because essentially unova is done and wrapped up. Doesn't make sense to go to a whole new place and still have it be part of a region that is done and over with.