To sum up!
Relevant bit of the definition of "violent conduct" is "a player who, when not challenging for the ball, deliberately strikes an opponent or any other person on the head or face with the hand or arm, is guilty of violent conduct unless the force used was negligible". Plus, it's been recently emphasised to officials that it can be violent conduct even if you miss.
The original charge comes after a review by 3 former referees. I think the appeal panel against red cards is a former player, former manager, and former referee but appeals against a retrospective charge seem to involve a QC and a member of the FA Council, and a former manager. You'd only have to have one of them thinking "it's no worse than a lot of incidents not given so much attention" and you might win the appeal.
Apart from those who think it wasn't worth a red card (at the time) or isn't "violent conduct" the main concern is that some United players (Rooney, Fellaini) commit violent conduct regularly and either get a yellow, or no card and no retrospective charge. Fellaini of course served a retrospective 3 match ban last season (remember Van Gaal's "It's not in the books that someone has to grab by the hair and then pull it behind - only in sex masochism.") Whether or not there is a bias, the key thing is that there is an obvious conflict of interest at the heart of the FA (and UEFA and FIFA): David Gill.
Finally, Marriner's failure to see it (or unwillingness to admit he had seen it but took no action) is bad, but worse (in refereeing terms at the time) is that he turned his back on the two players who'd clashed, potentially missing any further "afters".