Her dirty tactics were that much a problem for anyone and to be real most of the war was planned by Saurfang including a possible siege of Stormwind.
Honestly we see the Alliance support a guy that literally has for surname Trollbane and doubt he is going to change it for Trollhugger any time soon, why the Horde should feel indiferent against a faction that aside of Anduin and Jaina has a dark story before they formed this Horde organization
TBF, literally nobody was able to do so, heck Genn forgets to mention the blight so that way we can see Jaina do a “cool moment” at freezing the plague and flexing her muscles
Rokhan wasn’t really provided any reason to disobey Sylvanas. She wasn’t antagonistic to Darkspears or other Horde members. And the purpose of the gathering in the Stormsong was for everyone to witness how Baine admits to his fault. This way Sylvanas avoided mistake that Garrosh did by trying to assasinate Vol’Jin. Here everyone could witness that Baine being exposed and facing fair consequences, his imprisonment was lawful act.
Why would other leaders oppose it if it was fair and square? You help Alliance in times of war, you go to jail and tbf even the Valeera which is the spy of Anduin gets scrutined because of her race when delivering the meeting message from Baine(and both know each other from some time).
It’s already upsetting that I have to defend Sylvanas here, of all people. I wanted for Darkspears to be the first ones to oppose her, but she really didn’t give them a good enough reason. The only good reasons are the meta ones - that she wanted to control Darkspears (but never capitalised on that) and that she wanted to manipulate Talanji, and that she wanted to deal with Zekhan (but didn’t deliver on that either). That’s it. Vol’Jin didn’t start rebellion because Garrosh was attacking Alliance. Vol’Jin started rebellion because Garrosh was treating Horde terribly.
Also on this I think the writers in their poor try to not make it that much of a copy from MoP, they made the mistake to only show crash of ideas with only Theron and Baine which are often seen more playing with the other kids rather than their “usual friends” and honestly since this was MoP we needed to see the Screecher do something nefarious to put the Horde against her, even Thrall and Saurfang have to recognise this on that dreadful cinematic that is just before 8.3