This is about Irinlia after she came to the castle in Aldlyn:
Is it possible to change her approach later on? Because, honestly, it depends really on the country.
For example:
If it becomes possible throughout the game to get close to the princess of Begus and maybe even marry her this could lead to a very close relationship between Begus and Central, maybe even so close that both countries unite peacefully at some point.
So, no need to conquer them at all.
But that isn't the case for Amalgal and probably also not for Dorgania. For Amalgal the best long term solution might be to get Varea onto the throne through a intrigue or plot. That should lead to a close relationship between both countries.
Dorgania however has not only a King who suppresses the native races but also a bad male heir. The best long term approach might be to conquer them. But that requires a quite large and well trained army.
So trade might be a good option at the beginning but later on conquering might become the more important option. And every country might need a different approach.