I don't recall the dynamics of the Seanchan royalty being discussed in the books and I didn't read the encyclopedia that closely but it would seem to me that the Prince of Ravens title is just like the Daughter of the Nine Moons one, and would no longer be valid because Tuon is no longer the "daughter-heir" but has now ascended to the throne. I do not believe (again I have not read anthing contrary that I recall) that Seanchan has the sort of Monarch + Monarch's spouse/consort that Andor has. If this is the case, then Matt is the Emporer now (or maybe he has to declare himself such like Tuon did) but we really do not know the extent of his authority.