Whether Reid made a meal of it or not is irrelevant: Aguero will be judged on whether he tried to elbow Reid, not on Reid's reaction.
Marriner's claim to have not seen it might seem ridiculous, but actually it's not, and we'd be foolish to use that in Aguero's defence; just because someone can be looking at something, doesn't mean they necessarily saw it. This is true in every day life - staring at the TV and missing something, staring at the kitchen table and not seeing your car keys right there in front of you, etc. Human perception is a fickle and flawed thing. It happens to us all, and we're not in fast-paced, high-pressured position of referring a Premier League football match.
And as for these claims of some kind of Utd-media agenda driving the inquiry; let's not forget that Rooney was banned (and subsequently missed our semi-final against them) for swearing down a camera lens after replays were shown on TV over and over again. Fair to say that it worked in our favour that time.
The fact is this: Aguero was being manhandled by Reid and threw an arm out at him. It was reactionary and most probably wasn't intended to hurt the player, but you can't do that. Everyone makes mistakes - maybe Marriner made one, but Aguero certainly did, so we just have to suck it up and he'll have to learn from it.