Alliance being at fault does not preclude the Horde from committing evil acts. Things like Alliance aggression in Stormheim are simply facts. That do not prevent the Horde's reaction from being evil.
The traitors don't have an overarching plan though (at least yet). They are acting in reaction to individual events right now. Be it Baine rescuing Derek because his loyalty lies with Wrynns and Proudmoores or the rest of the traitor squad learning that Baine is going to be executed for treason (which, on a side note, is a punishment that Baine himself argued was OK for a Warchief to use against traitors). And in regards to those events, the player is told the plan. Actual Horde players, not so much.
And if that's the best Blizzard can do, they really shouldn't have bothered. Because the pay off is nonexistent. They couldn't make it more clear with their "play along" or stuff like siding with Saurfang giving a toy while not siding with him giving jack shit that the traitors will win and the "choice" to side with the Warchief is utterly meaningless. And when it comes the story the character experiences it somehow managed to be even worse, because the people choosing the Horde just get to experience the traitor's story anyway, rendering the very point of having a choice moot.
Derek isn't a fellow Forsaken, nor was his free will being violated (especially in a way comparable to Lich King's mental puppetry).
There are 15 named Dark Rangers alone, 3 times as much as the Forsaken you named. And Dark Rangers are the elite of Forsaken forces.
And how on earth does she further that by antagonizing Thrall and motivating him to return from his self-imposed Thanos-post-snap retirement? Unless you want to say that Sylvanas' power is the power of elaborate suicide.