This is just my opinion, not to offend anyone.
We are only one episode in and I prefer that Mary and Sebastian end up together. From the bat it seems as though he genuinely cares for her più than Francis and Francis seems to have forgotten everything that they had when they were little kids.
Francis seems to care for other things rather than Mary and Mary seems to Amore Francis and not just for her kingdom. Her feelings are real.
From the moment that Mary and Sebastian locked eyes, he was infatuated with her, he's falling in Amore and she could feel the same if she was not in an arranged marriage to Francis, who is having an affair with some girl, he doesn't want to o at least it seems that way that he wants nothing to do with her unless it pertains to business for them. That's just my observation, not saying that's what other people see.
So all in all, we'll see where they end up, the historic facts aren't accurate and I don't care about that, it's been detto that it's for entertainment and the Amore triangolo is somewhat the center. let's see where they go with this