Feng Ling Tian Xia
That's why I said "in a very religious society" by a "very religious person". It can be hard to imagine, but there are lots of people that would rather die than swear like that in that kind of scenario. For some people promises are air, for others they are all that matters. As for the second matter, the author never says anything, it's the princess who frases it that way and given the nature of their society and the person, they believe her. It's also neccesary to consider that even the narration is done from character's POV and that a forced oath is even less likely to work.