Grimmer can’t be given a ban as that would be unreasonable on him. It’s not his fault that the ref was crap.
The first incident is what has caused the problem: two quick fire fouls both, in most people’s opinion, merited a yellow card. So justice appeared to be done when the ref flashed a yellow at Grimmer, and then at Mawson too. But there was a bit of a delay between the two: words said to some players, but we don’t know what. I’m guessing, with the benefit of hindsight, that he told the players that he’d just booked the wrong player when actually he’d booked the right player (assuming that he didn’t think the first foul was bootable).
Did Grimmer think he’d been booked or not? There was no VAR like cancelling signal from the ref, but the message somehow got to the scoreboard operate as the yellow next his name disappeared.
So Grimmer commits another bad foul and get booked. Would he have committed that foul if he knew that it could mean a sending off? Possibly, but possibly not, and he can’t be banned for something that might not have happened had the ref done his job properly.