On two separate occasions I beat the main questline before starting the civil war. Once for the Imperials and once for the Stomrclaoks.
turns out, no matter how high your level is, the game will never spawn in soldiers high enough in level that Odaviing can't singlehanded win any given battle in the Civil War storyline.
Then there's the possibility of the Dragonborn showing up alone to the first invasion force and using Storm Calling to take out an entire army at once. When you can summon electrical storms that sent blasts of lightning to invariably target every living thing in the area but you, you don't need an army.
Edit: Ulfrick's shear hatred of the Thalmor is implied to because they have him convinced that he i singularly responsible for the Fall of the Imperial City.
Proof that they lied and proof that they're actively benefiting from his coup might make him listen.
Meanwhile, as I already said, there's world of difference between "being prepared for betrayal" and "knowing it's already happened."
The first is something to keep in mind.
The second is something to do something about right now... Like say arresting the traitor who is literally right there on the spot.
Either the Thalmor burn her and claim she and hers were acting alone and there invasion plans are delayed... Or the White-Gold Concordant is broken the Thalmor lose every resource they have in the empire as they either pull out or let their agents be arrested/killed.
Either way, the Thalmor's plans are severely delayed.
Edit: Weird brainfart.