Whether Henselt was killed, or if he was beaten at Vergen, I believe his main force was still massed on the north side of the Pontar. So those troops would have been in a position to resist the invaders for a while, if they kept their heads. . . .
"if they kept their heads. . . ."
Don't forget that all of the kings who formed the holy alliance who repulsed the empire during the second northern war have lost theirs.
Without their king and their allies of old I don't think the Kaedwen troops will last long against the hundred of thousands of black ones heading north.
Deprived of their morale they will likely run for their lives without even making a stand.
If we look at this with a clear mind the only thing standing between the north and complete Nilfgaard is Radovid.
In only five weeks we will asist to the clash of two mighty forces:
The severest nordling alive:
And the White Flame dancing on the barrows of his enemies:
It'll be a tale to tell the young 'uns