There is only one major injustice here. Zlatan's ban should be retrospective and back dated to the middle of February, cancelling out any goal scored, say, on the 26 February. I think United were playing someone that day!
As with any meting out of "justice" the outcome is judged not the action. There was a classic experiment some years ago when a group of people were asked to "pass sentence" on a guy who'd left his car at the top of a hill with its handbrake off. If in the story the car rolled harmlessly down the hill, people let him off. If it careered into a supermarket killing a child and her granny then they'd string him up. Similarly, stamping on an opponent is nasty. But it can occur without intent. So too can catching someone with an elbow when jumping for a ball. It appears that Zlatan intended to elbow. Given that, 3 matches seems the least he could get*. It is clear the jury is out on the stamping. Then comes down to benefit of the doubt.
* Obviously, for the offence of intentionally playing for Man Utd, compounded by scoring two goals against us at Wembley, he should be fined £4.2 zillion and banned for life