Building a road to the north would be their best bet to create a stable and prosperous society. Initially it would be a huge public works program that would put to work much of the displaced populace.

You're ignoring the most important geographical fact: the river is too fast and too full of rapids for boats until it merges with the other river from the north, near Remen. At the moment, a bridge in the vein of Tar Valon's is not an option. Even with channellers to help, they'd need to figure out the engineering first (something the scientists in WOT have little experience with - there's no attempt at big bridges made, the few that exist are all from the AOL or the early post-breaking).

So this would be a road to nowhere.

This is well established in EOTW by comments from Tam. It's one of the main factors why the TR has remained isolated even in more populated and prosperous times (even in the days of Manetheren, it was its rural countryside). Another one is that it can't be reached from Northern Altara either (from the east) because of the swamps area.

The traditional way to reach Ghealdan from the Two Rivers is through the mountain passes to and from the city of Manetheren itself. It's already in place, not used by Ghealdan has it had much better trade routes and trading partners.

Ghealdan, by the way, is one of the regions we've heard very little about, tradewise. It seems to be mostly rural, with little industry (we've heard nothing of mining in the mountains, for instance) and if it has specialities and luxury goods in demand elsewhere, we haven't heard about them. It's one of the nations (and about the only one) from which we've never seen any traders in other cities.

At the pace trade follows, it would be much easier to rely on channellers providing regular gateways in Jehannah to other trading cities. If the Seanchan stay, and remain antagonistic, Ghealdan is doomed anyway, probably the first nation not conquered yet to fall. The Seanchan borders with Ghealdan are soft, difficult to defend, and in little populated areas. If the war continues, they'll absord Ghealdan rapidly, so they're at least protected by the Mountains and the Manetherendrelle and the Woods from all armies coming from the north but those that Travel.