Nivemus planning on splitting is indeed a well known thing. I agree with Aeneas. The problem with the whole Sol and Swordfell was that Sol was literally called Fellish Duchy of Sol. Plus Crixus gave thorough instructions on everything. That is creating a puppet. Something you want to avoid in the first place. And doing that in the middle of an ongoing war....
Although you are in a war, the frontline is pretty far from Nivemus. I don't think OI and Nivemus are fighting directly? As long as the new realm is free to do whatever it wants, you are probably okay.
