I have just started the a project of multiplayer support for campaign. We can play Bannerlord with our friends from LAN.
I'm going to notice you about devblog in this post and my twitter account.
You can check it
The cooperative mode is my maximum illusion since always, I hope you are lucky and manage to complete the project.
For my part, I have read all the possible posts and opinions on the subject and if you need help to focus on various problems (such as real time or starting speed) I can try to help you if you wish.
Also if you need to test the content, I can help you too.
As key points, I think the anticheat system is something you don't have to worry about because, as I have seen, people want cooperative to play with their friends, not to have a "Multiplayer Campaign" (which would be nice, of course, but not the initial intention).
As I have seen, and I can be wrong, the general idea that users present is a cooperative system that allows, in the same game, that the host and a friend can each have their own army. For example, I conquer city A and my friend conquers city B or protects me or helps me conquer city A.
Playing as "partner" I think it would be tremendously boring because everything depends on the host, the best would be to try to find some "independence"
I do not know if what I present will be possible, I trust your criteria and skills.