Mountcastle had a good year. 6th is defensible. Defense matters too. Higher would have been defensible too. I'd maybe have him over A. Garcia, Clase, and Franco. I think Arozarena was a reasonable choice.
While I agree with your points here, I don't think 6th is defensible.
Garcia had a .626 OPS in the second half. He was in free fall.
Franco didn't play half the season. If he plays the whole season, he's the RoY, but that's on the Team, you don't get credit for doing it 'that way' unless you do something truly special with your 70 games. You can't be good and then project it out.
Defense in this group of players doesn't really matter. None of them are special defenders. I disagree you can say "he's a better defender, so he gets some significant bump." Mountcaslte gets paid to hit and he did it, over more of the season, better than any of the others. ...and if you want to start including dWAR as a metric, that is hugely problematic given the wonkiness of that approach.
I had Arozerena on both of my fantasy teams and I was frustrated by how often they would rest him. If he wins, I think THAT is justifiable, so RM should have been 2nd,
You can make a better argument for Mountcastle winning the award than finishing 6th.