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.
Well then they need to re-train their referees and put forward the notion to be extra vigilant then, as someone's safety could depend on it.
Imagine that "I didn't see him smashed in the temple despite being 4 feet away because I was looking at it but not paying attention". Inept.