The Stormcloaks are the only soldiers who will actually fight against the Thalmor. Imperials will side with them, if they see the fight break out.
Ralof says: "What the rebels like to forget is that the Empire is what's keeping the Dominion out of Skyrim". Yeah, right. Bang up job they're making of that.
Also, you can't accuse Stormcloaks of "betraying" the Forsworn, because they never had any agreement to begin with. It was the Empire who betrayed the Stormcloaks, on that occasion.
Only if you take a needlessly roundabout route. The most obvious and direct road between the cities - goes nowhere near Darkwater Crossing.
Thought: there is a fort, whose name escapes me right now, a little to the west of Darkwater, populated by dead Stormcloaks and live necromancers. Maybe it was the Imperials who killed the garrison, as part of preparing their ambush for Ulfric, and the mages moved in when they moved out.