Ideally, fiefdoms joining forces would make sense, however, from game design point of view I'm not sure how to tackle it given that in VN mode players can conquer any fiefdom they want in any order they want and even in normal mode I can't know which fiefdoms will be conquered and in what order.
An alternative to joining forces would be something like "reinforcements from the Western Sister", where each fiefdom's strength grows with each each of Karder's conquests (of course, they'll need to still be beatable...). I could apply this after the 5th of 6th fief, so not to break the existing status and this could also tie nicely into the story, with a cutscene of the Wench with Queen Remy.
Oh, and I didn't think of any of this until I began to reply to your message, so thanks!