Blaming Rastakhan for what Zul did during Cata/MoP is like blaming Terenas II for the Scourge. Rastakhan sent Zul off with a fleet because he was just about done with Zul's prophecies of Zandalar sinking under the sea. Also worth noting that the Alliance and Horde fought off the Zandalar together in both expansions. You can't call yourself peaceful and just when fucking Sylvanas takes a more diplomatic approach than you do. Though I do agree that Rastakhan is a dumbass. He's a fairly good-willed dumbass though, selling his soul off to the Loa of Death to save his people from the blood trolls when he FINALLY realised they were about to run his kingdom over.
The Horde quests involving Garrosh and Sylvanas are actually not as bad as you'd think. The PCs consult with sane leaders and individuals of the faction, observe just how far the warchief is willing to go, and take part in an uprising to be rid of him/her when it becomes certain they can't be redeemed. Not the best story ever written and it certainly wasn't worth repeating, but it's a true demonstration of intellect compared to 'look Nazmir's a bit misty, let's throw the entirety of our armed forces at it and leave our home base completely unprotected!'.
The Alliance has spies setting up bombs in Zandalar before Jaina is freed from Gorak Tul, pretty sure this is before Rastakhan's army sets foot in Kul Tiras. The Alliance has indeed also made a fool of itself now and again but afaik the Alliance player was not responsible for such events.
Again, there is a difference between Alliance NPCs looking like idiots and Alliance players looking like idiots. Sure the Alliance leadership hasn't been the brightest (though I'd argue Horde leadership has generally been even worse since Cata) but the players themselves seldom act like morons - at least not in the expasion we're talking about. Horde players are literally one of the two responsible (the other being Talanji) for being played like a fiddle by Telaamon and leading the 'entire' Zandalari army into Nazmir resulting in the events of the Dazar'alor raid. It's a whole different experience when the game makes YOU the fool rather than having you clean up after fools. Last time I saw this kinda shit happen was with Jin'do back in vanilla, but he a) screwed over both factions and b) was actually crafty enough to make HIM look badass rather than making the player look like an imbecile.