Ordering the saintess to die to revive someone that killed herself because she didn't want to go forward with her arranged marriage is really dumb. First of all, you're losing the saintess, who could be used to heal life-threatening injuries numerous times instead of just dying to revive one. Second, you're trading the useful saintess for a useless princess that refuses to do the one thing a princess is expected to do: get married for the convenience of the country. So the royalty of that past kingdom traded a useful person for a useless person.
Also, we don't know if this was planned together with her fiance or if the princess did it on her own, knowing that if she killed herself the Marius guy would be available. I hope it was on her own, but it's dumb either way. If Marius was in on it, then what exactly was stopping him from just refusing to get married to the saintess? Why would you need to kill her to be rid of her? Surely he just needed to tell her he loved someone else. The saintess seemed to have no other standing that "being the saintess", or else her family should have protested against her sacrificing herself for the princess. But if she was on her own, then even if her father backed down on the unwanted marriage, would she have any chance to snare Marius? She would have killed the woman he loved out of selfishness. Why would Marius then want to spend any time with her, much less marry her?
As for her current plans, it makes sense. If you know you have so much power and you already lead a comfortable life, why draw attention and let those with more political power take advantage of you? Better to keep your cards close to your chest and only use your power for yourself and those you care about. She seems to have a loving family this time around, screw the royals.