Well, I also have a theory of my own. Because they exist in the same megaverse, and if ACOTAR really did happen before ToG, then I believe that all the species have evolved into species from ToG. Hybern, being as they are, they could have evolved into the Valg, and moved to a different dimension from which they come out of in ToG. The Fae obviously stay the way they are, and mortals also stay the way they are, but they both slowly change and develop. Like the Fae, instead of having 7 courts, could come together as one in wendlyn like in ToG, but the mortals spilt off into different kingdoms/queendoms.

Also, if you think about it, if the ACOTAR World was like the Pangaea of SJM’s world, then it split off into different continents in the ToG world. Prythian became Wendlyn and the other kingdoms with magic in them. Most of the Fae could have stayed in Wendlyn, because that’s where most of them are.

I think this could be possible, but I’m not sure. Again, just a theory.