I think what irks me about the first goal is the line that "because he didn't blow his whistle before the ball crossed the line, he must give the goal". If a player commits a foul, say climbing on the back of a defender, while heading the ball in to the goal. The ref will rightly blow the whistle for the foul, after the ball has crossed the line. So where is the difference here? If the ref believes that the game should have been stopped (which 99/100 it will be for an injured goalkeeper), but he hasn't reacted quickly enough, to blow the whistle, why can't he pull the game back after the ball crosses the line?
I'm not saying that's what happened here, maybe the ref didn't want to stop the game in this situation for whatever reason, but this hard line of "the ball crossed the line, so it makes everything that happened before then moot", is what I find frustrating.
I hope De Gea has learned from it though.