Yeah, the moral relativism argument isn't going to win me over. For one thing, apartheid and slavery are simply never justified. Second, it's not nonfiction. It's a fantasy novel meant to be consumed by modern audiences. There's an expectation that we'll read characters through our own moral lens rather than through a fictional system of morality.
Your second point is fair enough and valid. Moash's blind pursuit of vengeance over justice leads to his own fall. Perhaps that's the difference between Kaladin and Moash. However, contra Moash, I'd like to see Kaladin work to change the system from within in that case. As it is, I worry a bit he's come to accept his new place in the hierarchy a little too readily, though there's obviously a great deal of anxiety there. That he's unable to choose and ends up breaking down rather than simply picking a side when Moash and the parshmen surround Elhokar and the Wall Guard because he saw that everyone was justified in some way is probably the best indication that Kaladin is the best person in the series, even if it nearly cost him his life.