No, the north wouldn't. Thomas spent several real life months arranging the Highmarch/Fallangard treaty with the north and getting Caligus onboard. Had Fallangard accepted the treaty, it could have walked away from the war losing nothing and it wouldn't have been under any threat from the north for the duration of the war. Once the war is over, Fallangard and Caligus would then have been left to sort out the issues between them in what ever way they saw fit (that was written in to a previous version of the treaty, but I can't remember the wording. I rewrote that treaty so many times....).
Obviously, I can't predict whether Caligus and Fallangard would have ended up fighting again. Relations between Fallangard and Caligus might have improved under influence on Caligus from some of the other northern realms. But Highmarch was incredibly protective of Fallangard throughout the treaty negotiations, so Highmarch would probably have sided with Fallangard in a Fallangard/Caligus war. But it's hard to predict these 'might have been' scenarios.
Fallangard made a huge mistake in rejecting the treaty (I know they probably don't see it that way, however) and in then burning their bridges with Highmarch.
Aramon was in support of giving Fallangard an easy way out of the war, and would have likely supported them later if Caligus had moved against them, given that they're historically allies of Greater Xavax. If Highmarch and Caligus can bury the hatchet (even temporarily) given the history there, there was hope for Fallangard and Caligus.
