I think they didn't have much of a choice, get played or get stuck where they are, the great wall he build/fortified left them with one route, and they have to close up the north otherwise even if they invade kantan the royal zhao will just escape the north, so not much of a choice they had except aim for the north first.
I agree, I don't think Kanki will make it easy for Riboku, maybe even Ousen follows up behind the scenes like gathering all those reinforced that are chased back to Qin.