This in every way. Anyone who's ever played football knows what happened. If you're running at full pace and an arm comes towards you from your periphery, you raise your arm instinctively to block or defend yourself. In McRorie's case, he even tries to get it over/round the player too, as you say. There is no malice, no arm where it shouldn't be or any of the other bollocks that people who watch the EPL or pundits will tell you, it was all entirely natural. He wasn't "a bit silly", a "forearm smash", or any of the other cliché'd pish, it was an entirely involuntary action, caused by the defender. An unfortunate collision in a contact sport, in a tenth of a second. Most importantly, it wasn't "out of control", or a well documented situation like jumping for a header with your arms out, McRorie was the player being tackled. It was, without question, a defensive block rather than offensive. That is what the referee, and everyone in the stadium, saw at the time of the incident and why the referee chose to act the way he did. He didn't miss anything, he wasn't confused by what happened, he saw exactly what occurred with his eyes and interpreted it as above like everyone else.
Then VAR intervenes. That's the point where all context is lost. We're now being asked to analyse something completely different. We're now being asked: "does this player make contact with Dunne's face?". The focus is changed, the picture slows, and you're left with a red card. It warps the thought process entirely. All it takes is for the referee to go to VAR without a clear picture of what he saw and how he interpreted it in his mind, and he'll change it. That's why Collum didn't change his mind in that Hun game, because despite what we may think of him, he has the courage of his convictions and he went over to the monitor with the attitude "I gave a yellow card because this is what happened when I watched the incident". Irvine was a weak referee, throughout the game, who would never have had the decency to say to VAR: "well that's not the way it happened in real time". Because the images might be the same, but the context most certainly isn't, and that's the most important thing. It's the equivalent of me making a homophobic comment about the referee quietly to my mate who would understand the satire involved, but having it secretly recorded and played back over Twitter to screams of "ban him for life!" (that didn't happen by the way!). When you remove context from a situation you can make it seem like anything has occurred, it's extremist behaviour, and football is just mirroring society as it scrapes the barrel of surveillance looking for offence.