Not hard to predict the future here.
ML loves FL and is 100% authentic, the bet the ML has with the king is that if he's able to woo the FL he'll name him crown prince, which may or may not cause drama later.
Well first off, "moving on" when you've been together long enough becomes very difficult even for humans, for a being that's literally immortal the idea is probably excruciating, there have been many cases in history of this very thing happening with humans, much less in fantasy epics.
Also...
I'm really glad that kazuhiro is worrying about his lifespan in comparison to marie's, too many stories ignore the implications of a immortal elf and a human being in a relationship (which is funny since tolkien solved this as elves marry for life, which also means they normally die of...
Da ji, ah yes the arguably most villainous female in chinese mythology and likely the inspiration for the villainous 9 tailed fox of japanese folklore.
The point is to secure the hostage before they can kill them, that's why mei went bulldozer while the mc went as stealthily as possible, bulldozing through without a plan pretty much guarantees the hostage is either gonna be taken or killed.
You'll notice another thing about cain's fiancee's, all of them are either royalty or some other form of important individual aka the saintess, it's possible that polygamy is rarely tolerated in esfort, but considering the politics involved here, it gives them justification for marriage to cain.