If you want to claim that the Alliance rebuked the Forsaken
because of Garithos, then it's up to you to prove that. To my knowledge, not a single Alliance member has referred to this event at any point, and given the relations between the factions, this seems like the kind of thing that would be mentioned had the Alliance known about it. Unless explicitly said otherwise, we can't assume anyone actually knew of Garithos' death at the hands of Sylvanas, this means we can't assume the Alliance can use Garithos' death as justification for killing these emissaries.
In fact, there is some evidence to suggest the opposite. In the Ashbringer comic the Alliance refers to the Undead in the city as "not a threat", logically this means that the Alliance didn't know the Forsaken were responsible for killing Garithos and his men, otherwise they would've viewed them as a threat.
Occam's razor is with @
Mehrunes here. The simplest explanation is that Garithos and his men died, and no one in the Alliance ever mentions how they died because they don't know about it.
Bottom line is that Sylvanas sent ambassadors everywhere, the ones sent to the Horde either returned or were accepted, the ones sent to the Alliance were killed. From the perspective of Sylvanas and the Forsaken, this sets up the Alliance as a hostile nation that cannot be negotiated with.