Okay, maybe she did expect it. That only makes her more insane or more evil, depending which you prefer. She slaughtered the last remnants of Lordaeronian humans and then expected humans to come help her... wow... just wow... . One would think she might have had a better chance of getting that support by making a deal with Gari's troops, who hated him too. "We let you go, you tell our story and we work together." But nah, slaughtering them all and then sending shambling zombies out to beg for aid seems like a much saner option...
That is decades later, when she had long established her choke hold on the Forsaken and they are long a part of the Horde and thus more or less working with other races.
Of course. No one cares about that racist SoB. His troops are a very different story though. I am not just saying it was evil and hypocritical that she murdered them all, I am saying it was incredibly stupid if she wanted a relationship with the humans. Those folks would have presented at least a chance. Make Gari's death look like an accident or the work of Scourge, ally with his troops, who hate him, send them to SW to make Sylvanas' case for her. Sooo much more sensible.
Nowhere is there any evidence that Garithos planned to break the deal. He only told them to leave "his" city, which by the terms of the deal was exactly what was supposed to happen. The betrayal was completely one-sided.
Is it possible that Gari would have turned on them later? Absolutely, but he never got the chance and we have no hints other then his character and commitment to being an asshole to support this theory. On the flipside, we know that Sylvanas was going to break the deal anyway and the character of the human commander had no bearing on it. She barely knew him. Her decision to kill all those humans was made long before and she would have betrayed any commander.
As for pushing for Sylvanas being evil. You know, not really hard work with her when you just lay out the facts. They kinda speak for themselves. I would say trying to explain away all the evil shit she did to make her look less evil is a lot harder.