I´m still beeing a merc clan and noticed that Lagerta rebelled and had only like 100 garrison and 150 milita. So I left the kingdom as a merc and tried to conquer Lagerta. I have 150 troops in my main party and 90 troops in my companion party. I tried to besiege it to create my own kingdom but had a lot of losses. And it doesn´t make sense for me that I can´t get my companion party to join my siege. While I´m sieging they just wander around the map doing nothing. Lazy companion...This would be a good one. I tried creating a small patrolling party with my son leading it, but he was constantly called to some AI army.
And yea, as soon as I become a merc again my companion party will always be in an army.