Event Pokemon and Pokemon holding Mega Stones can't be traded over the GTS. You'll have to do a link trade if you want those done. Warning: Festival Plaza sucks when it comes to link trades if the person isn't already on your VIP List.

Gen. 7 TIDs are derived numbers and can't be edited directly. Many different variations of TID/SID can form the same Gen. 7 ID. Which is why if you want someone to edit your Pokemon to have your ID you will need to give us another Pokemon caught or bred in Gen. 7 first so we can determine the TID/SID.

Sources: https://projectpokemon.org/home/forums/topic/39531-pkhex-for-sun-and-moon-trainer-id/

https://github.com/kwsch/PKHeX/issues/490