This is true. I am probably biased though as the Mac Allister goal lifted me, the crowd, the whole atmosphere to another place. That's why it hurts, especially as the incident with Mwepu seems very slight and at the least debatable.
You are right as teams adjust tactically according to the situation they find themselves in. Retrospective var cannot be a thing. Except perhaps in the case of man u being awarded and scoring a penalty after the final whistle had gone against us.
The Skalak volley was insane. However, when I wondered about that yesterday, I rated the Mac Allister one higher as the level he's playing at is higher.