in the event that the north(pre conquest) managed to conquer and subdue the vale to a point that they kept control over it over the generations, do you think that the situation for the mountain clansmen would become much better under the rule of their fellow first men?

In the event that the North had won the Worthless War and conquered the Vale, I think the Mountain Clansmen would have seen a substantial reversal of fortunes. Keep in mind, the Worthless War came pretty soon after the Andal conquest of the Vale, so the North might well have confiscated the lands of the conquerors and returned at least some of them to the mountain clans who had been dispossessed.