If you and an opponent each have 11 cards in your hand, and you play Trade Hands, would you win because after the card’s effect occurred you had 11 while the opponent had 10, or would the opponent win as soon as the card was played and left your hand?

A: You should need to finish the Action (i.e. follow through with the hand swap), before considering the issue of someone meeting the Goal. Thus, it’s that first thing you said.