About Sylvanas, I don't really see where this "honour" is. He just kept an eye on a potential enemy, since he didn't trust her, nor the whole of the Forsaken. The reason for which the blight was almost banned is because of the events occured on the Wrathgate, not really for questions of "honourable combat".
In Edge of Night he planned to use the Forsaken soldiers as easily-expandable meat-shields, because of their unnatural resilience, so that his awesome orcish soldiers would have been all rested and full of energies for the final, and FAR safer, assault. I don't know where is the "honour" in this and which kind of relevant meaning has bitching about the plague behind the excuse of the "honorable combat" when you're ready to treat your own troops in such a way.
Ofcourse, if we're talking about the practical, tactical angle of the matter, exploiting the Forsaken in that way was pretty smart. But so it was the whole plan for obliterate Theramore and isolate the Night Elves from the rest of the Alliance. But that's just prove my point repeadetly: Garrosh values victory above all else, and every mean justify that end.
There has never been any slight of balance, the stuff putting Garrosh in a very suspicious/ambigous light heavily beat the rest. If anything, suddenly turning Garrosh in a somehow positive character, after all the stuff transpired between game, novels and short-stories, would have been incredbly far-fetched.I expect they were trying to strike a balance but ended up making him still seem like a douchebag so in Mists they went full out.