This mountain problem is no one's fault but Darka's. If Darka would walk through Cantril we would be in the CE without the problems of the mountains, but Darka refuses to offend Talerium for some crazy reason.
It's not a problem of just going through "the mountains". It's a problem that the one-region-wide route allowed by the treaty leads straight into a fortification. Aside from the border with Carelia, CE now only has one border region, and it's fortified.
Darka's policy has been to not piss of Talerium for as long as I've been aware of even the basics of Darkan policy, so this is hardly new.
So, in effect, Coria gets out of the war, CE gets a fortified border, and none of the northern realms gain anything of note. That's a really,
really good treaty for Coria. I just don't get why Eston agreed.