I found a potential solution that I made into a thread. https://inheritance.wikia.com/d/p/3308739104866332005 I also realized that the only ones subject to Nasuada's rule of Magic are those in Broddring Kingdom.
I know that, it was a multinational agreement. Part of the terms for letting Teirm be independent was that they agreed to Nasuada’s mage plans, and Surda’s land acquisitions also included agreement as well. Surda and Broddring are allies, and as such have the same mage terms. It’s in Inheritance.