Responses
Well, Arth and Sasha do get married in the end and have two children. Rebecca broke off her engagement and inherited the duke household and never married. As for her successor, I think she'll adopt from a branch family. From what I know Rebecca's ex-fiance never inherited the throne(quite obviously) and payne did instead. I'm not sure about naska though

even though i saw spoilers from the novel. I'm still going to need a story route for Naska and Rebecca