Any creature the player controls that couldnt be targeted by the original spell (due to shroud, protection abilities, targeting restrictions, or any other reason) is just ignored by Mirrorwing Dragons ability
Trample Whenever Grim Flayer deals combat damage to a player, look at the top three cards of your library
Ascendant Spirit doesnt solve this problem per se, but it does have a lot going for it
In the meantime you can read my full Q&A with Wizards of the Coast down below, or if want to see some more fancy showcase versions of cards from Magics most recent set, you can watch me unbox the Phyrexia: All Will Be One Compleat Edition bundle
It will cost you $16 to sleeve a constructed deck with StarCityGames Art Sleeves, $10 to sleeve a constructed deck with Legion art sleeves, and only $8 to sleeve a constructed deck with Ultra Pro
"Magic: the Gathering Tournament Rules" (PDF)