In my playthrough Birgit keeps switching sides from one kingdom to another. I was quite surprised to see this, given that she's heir to Otto and all. However I think it is quite easily explained by her losing all settlements and having the martial personality. You were supposed to help the Reich become über alles princess, not join the fricking Zann! Meanwhile I'm having a götterdammerung hard time keeping the sole city of the Reich from falling into the hands of whatever faction they're "fighting" (as in getting totally wrecked by) at the moment. The brave lords of the Reich have about 20-30 low tier troops on average and thus get repeatedly gibbed by deserters, snowmen and Illicans. I don't want them to die out completely, since out of all the factions, I like the look and theme of their army the most.
In other words, I can't wait to play this mod again, this is getting so exciting! Maybe I'll hold out long enough for Otto to appoint me marshal so that I can take Sut and from there onwards, recreate the Miracle of the House of Brandenburg in Perisno by slowly regaining all of the Reich's lost domains. Maybe the Zann or Drahara will finally swallow up what remains of the Reich finally and it will be Three Kingdoms. Maybe I'll become Zann's greatest champion, help him take over the entire world, only to turn against him at the last moment to instigate my very own Horus Heresy.
I really wish though that severely weakened factions were to be given some sort of bonus to give them a chance to recover and not die so quickly from a Blitz. In my playthrough, the Reich went from being semi-viable to dead without player holding its sole settlement in a span of about 20-25 days.