Edit 2: Litwick appears as an item in Photo Booth. Might be in the dex, in which case Island Scan is going to change a bit.
Edit 3: Swirlix appears as a photo frame in Photo Booth. Gorebyss' name appears on a trick for Mantine Surf.
Edit 3: Manectric is facing off against Sandslash. Houndoom is facing off against Ninetales.
Stuff like the photobooth and names shouldn't necessarily be taken as proof. The most obvious examples are the pikachu billboards in BW1/2 and Torchic for Amity Square in Platinum
SM in particular has Krabby on the Tide Song Hotel (as well as Horsea, an island scan pokemon).
So trying to math out the mega pokemon rq....
Base Alola Dex had 17 lines of the 45 lines with megas (ralts line has 2, obvs)
USUM has thus far shown 5 lines (btw you have larvitar listed twice)
There's 6 starter lines with megas, who probably wont be available.
Another 6 legendaries who probably wont be in the dex (though i'd be surprised if latios & latias weren't post game)
So that's 13 more lines if they wanted all of them available, which would come out to be..another 29 pokemon, I think. Seems pretty feasible and still leaves room for assorted randos like the meinfoo line and inkay line
as an aside this stupid piece of artwork drives me nuts
I know literally it's accurate since any QR code could show any pokemon but they could have at least made all 3 island scans. It'd be nice in general if island scans got an ~ultra upgrade~ to allow more pokemon than the 84 we got. At least let us reach 500 pokemon!