Personally I limit my gaming experience until vassalage.
The world is so huge that conquering everything seems quite a unnecessary challenge at this level of development. I will consider it after full release.
I'm also enjoying a different gaming experience using Freelancer mod.
Regarding the truce proposal system.
The thing is that AI is taking decisions based on different factors and ratios, and the player is only considering the last won battles.
Mexxico somehow tried to give
some examples here.
Maybe in your case, your kingdom is ready to pay tribute because the war with Aserai is not interesting for them, and on Aserai side, calculation results in some opportunities to expand their territories.
As you said, the player lacks of visiblity and it easily leads to misunderstanding. In the coming updates, they will improve the information to the player by giving more details about
the reasons of war and peace.
Companions are usefull because you can create extra parties as well. And your clan does not have a party cap (3 for AI).
Some players will use this system to create an army with only the companions parties, for even more rewards (renown, etc.).
You can assign them as governors too, really usefull to avoid rebellions for example.
And of course, you may use companions in your party to get benefit from their skills (surgeons, engineering,...)
The only negative thing is that they tend to die quite quickly on the battlefield (but now we have an option to reduce the death % for companions).