Pointing it out in case anyone misreads your post as my having said what you’re quoting.
Sylvanas and Genn have a history. Genn and the Forsaken have a longer history. Or do the Forsaken not get the same right to hold old grudges for the way he abandoned them to the Scourge in the 3rd war?
As far as I know Sylvanas has never brought this up. It wasn’t in A Good War, and while posters here bring it up from time to time, it hasn’t been part of the character lore presented to us by Blizzard. Oddly, the only person who has brought up Greymane’s wall was Varian in Wolfheart when Malfurion had to convince Varian to let the Gilneans back into the Alliance. Sylvanas has never brought it up.
But that has no bearing in universe or canonically for the assassination attempt at Stormheim.
We covered that earlier as well:
His main motivation was:
Genn Greymane: You took my son's future.
Which happened because Sylvanas tried to assassinate Genn.
So once again, all answers lead back to Sylvanas.
Genn lets his emotions rule and decided that by retreating and sounding a call to retreat (in the cutscene no one had any doubts as to what the horn signaled, so it was given to them as well) that the Horde has broken the treaty and that he must punish them.
Once again, if Genn believed the treaty was broken, then Veloran’s point is wrong. And once again, the rest of what you said isn’t relevant to the conversation at hand.
The big problem with the whole Horde is fighting for its survival theme is that it seems like every time that the Alliance does something that threatens the Horde it gets retconned or explained away later in the story. Or just ignored.
Got a whole different thread for you already:
TD;DR:
Sylvanas: Look how much of a threat the Alliance is!
Anduin: Lets save a Horde leader!
Jaina: I’ll do it!
Genn: What?!
Jaina: But Genn…! Come on!
Genn: Okay. Lets work with the Horde.
Lor’themar: Jaina and I are friends.