I think we've all taken part in a physical sport where we have the potential to lose our cool. A few months ago I had a player elbow me in the chest whilst I was marking him on a corner. I got in his face after the incident, but kept my cool because I knew the official was watching. Ultimately, the lad was shown a straight red.
The difference is how one reacts to the abuse. Fellaini needed to keep his cool and not do anything that would have gotten him sent off. One incident I could understand, but the slap minutes later made his actions just selfish, reckless, and stupid. He should know better that his team can't afford to go down to ten men in a match. I'm an American so I haven't seen MOTD. These are simply my observations after watching the match without pundits. Fellaini should've brought the issue up with the official himself or through Jags. Shawcross was all over him during every set piece, and perhaps pointing that out would have drawn the official's attention to it.
If nothing occurs, I'd probably throw a legal shoulder into him or ensure that a well-timed tackle sent him tumbling. Something that will get Shawcross pissed and increase the likelihood that he'd do something rash to get sent off, but I'd only risk earning a yellow at most. To do something twice that not only could've resulted in a red, but also a significant ban, is incredibly stupid.
Credit to Fella for apologizing after the match, but I'm afraid it won't save him from a ban.