Yep - again see your point & the league one example is a really good one, but as I said re Ziyech; to the best of my understanding he would simply be ineligable to play for new team during this season irrespective of when it is played. He is not registered and while he might be a player for his new club, he cannot play in this competition.
& again, you are right it is a legal minefield, but what I have argued since day one in this thread, that every outcome results in a legal catastrophe & the "just void & be done with it" is equally as complex as what you have raised above