VAR should be looking at both potential red card incidents. The first because it's a potential red card for denying a goal scoring opportunity - so it's irrelevant if it is outside the box. The second because it's a potential red card for a foul. The fact the ref gave a free kick and a yellow is irrelevant if he does so in error.
What is relevant is whether the ref saw what happened and his decision is based on that. If he tells VAR he saw the arm to the back of Mac Allister's head, but felt it wasn't enough, VAR would have no reason to refer him to the screen. Similarly if Ref saw the studs to the achilles, VAR would have no reason to refer him to the screen.
The question then is, if the ref saw the forearm smash to the neck, and the studs raked down the achilles and doesn't deem them worthy of a red, what does he view as worthy?!