I think the novel which they read was written by Fake saint, she manipulated the story that the duke (and prince) loved her enough to revolt against empire and intentionally recommended it to fl and ml, so that they would think that ml/duke was destined to love fake saint.
Maybe that's why there was no mention of other Espars and guides, because fake saint never completed her quests.
I'm still confused with the time line, when he went in the future and crossed dimensions he met fl, but now they both returned to past and fl is in this world, so does that mean she is not in korea now? or maybe in previous time line she died in stern empire and lived in korea, and in this timeline she lived in stern empire and died in korea.
If your bloodline is cursed why have baby and pass the sufferings, well I shouldn't use much logic (〜 ̄△ ̄)〜
id assume being so consumed by love for someone it becomes part of the curse. loving them so much you keep having children despite the curse
It's also possible that since they're a noble house they'll need to keep producing an heir and stuff.