Not true. The Boltons have not united the North, they have simply made many houses too afraid to act. Most of the houses turning Jon and Sansa down are not going to be fighting against them. They just don't want to fight, period. In an oppressed state, if the Boltons were to call to arms against the White Walkers, they would be sorely lacking in support. Without the Boltons, and with a true Northern house leading the North again, there would be more troops, higher motivation, etc.
As for suggesting the Starks join with the Boltons.... roflmao.