I wonder how this will affect Cass, or if she even cares.
I have the impression that the Pentaghasts has a whole do not really care that much about Nevarra, it seems to be more of a buffer against Tevinter and Orlais. Even after the 3rd blight, the Pentaghasts liberated Hunter Fell (taken over by Tevinter) decades before Nevarra City (taken over by Orlais). They seem to prioritize their ancestral home a lot. Now, they have a succession crisis and, going by the no known children and really old king, it was known years ago that it was going to happen. It almost as if they let it happen actually...
As for Cassandra, I don't think she really care who rule, she left politics and her noble life behind years ago. I even wonder if she plays The Gametm in Orlais. Leliana said everyone was playing it, but Cassandra doesn't seem the type to backstab and manipulate others. Punch someone hard in the face, yes, plotting in the background, no.