Well, sorry if I may have missed something, but there's something conflicting that I can't get my head rest from. I just hope I missed a spot check in the whole story.
Timeline of XY - SuMn. There's something that bothers me. The backpacker in a motel gave us the Strange Souvenir, highly implied to represent the Tapu (it's sold in the megamart as well). And he also mentions the extraordinary Champion of that region (while specifically saying he's not from the 6 other regions). However, we know Pokemon League wasn't built yet and there's no Champion at all until Kukui did it as SuMn story progresses, implying that XY happens after SuMn.
And then in Alola we meet the two kids, Sycamore's assistants Sina and Dexio. Before handing us the Zygarde Cube, they referenced the group of five trainers, obviously referencing the XY kids, "in the past". This implies that XY happens before SuMn, since before the event of XY, they were yet to be trainers until Sycamore give them their starter Pokemon, starting the XY story.
Now, assuming they're in the same universe of Mega-timeline and they made no error (lol), which is true?
Does XY happen before SuMn? Or does XY happen after SuMn?
Or is the backpacker actually not referencing Alola, opening possibility of different origin of Tapus than thought or that there is more than one Tapu of their species? Or is Dexio referencing another group or a retconned story arc? Or... what?
IIRC There are multiple mentions of Island Champions existing before Kukui decided to create the Pokemon League, so perhaps he was referring to them in XY, just shortening the term to just "Champion".