At some point, a nationalist/neo-monarchical movement springs up within country B, as they contemplate the good ol' days back when territory C was part of country B and they had a monarchy (even though "those days" were pretty terrible).
So, there's both a big push to reestablish the monarchy and to reunify territory C and country B. Luckily as a result of A royals and B royals mingling the monarch of country A is also the "rightful" heir to country B.
Great! So in an ideal case scenario for these B nationalists B and C would reunify and the monarch would rule A and B in a Personal union. P…