Impossible that the Zeal Royal Family had anything to do with the Guardian Line, simply because after the Ocean Palace disaster, there were none left. Well, at least in the original timeline, Queen Zeal and Schala are MIA, and Janus is sent to 600ad, and when Crono interfers, Janus is in the DBT, Schala merges with Lavos, and Queen Zeal is in the Black Omen.
I selected 'other'. Don't think Inheritance is passed down based on gender, rather I think its passed down by age. The eldest child, regardless of gender, would be heir to the throne, and heir's future husband/wife would be married in.