iirc it was a book that ogSelonia saw thinking it was black magic? idk
Hah! I knew it! Grace was the ogFl! I guessed that as I was reading. Like the maids were saying how sellonia became from an angry person to a calmer one. Plus she easily got mad when the MLs were slightly breaking from her spell. My theories being correct aside, she really is annoying to do all this cuz she's lazy. And about her father, I was surprised that he's also guilty along with his daughter however it makes more sense as in one of the scene he was talking to the dude who challenged Tan (i really don't like this name; it just doesn't do justice to ML), he seemed really cold and did not have the sweet demeanour he has in front of FL. And as for the informant's attitude, I thinks it's safe to conclude that he's in cahoots with the ogFL.
According to novel update spoilers this is only revealed way later in the story, but "Grace" is the ogfl and she knows about the future/novel content (the person who gave out the spoiler said they forgot/didn't notice if ogfl was a transmigrator, a reincarnator, or just learned about the future/novel content). Because she knows that she will have to go on the demon king subjugation, she created a plan to use black magic so she can safely swap her soul to another body. It is presumed that this required 100+ human sacrifices to do because if they wanted to have "Grace" get back into her og body, she will need around 100 human sacrifices. Her entire reasoning for this was because she wasn't willing to suffer from camping on the ground, get hurt during fights, and eat monster meat during the journey.
Her father is in on the plan and funds "Grace" so she can do whatever she wants, and the reason why he is treating mc kindly/keeping her body healthy is so the body can eventually go back to ogfl. Also, this is already apparent but "Grace" is using some sort of black magic to brainwash the three guys and if I recall. She did all this because she believes that because she is the ogfl, naturally they all should all belong to her (something like that?).
MC will eventually cure the black magic that brainwashed the 3 guys, and the paladin is truly regretful for his actions. The dragon also regretted his actions but doesn't immediately apologize because of his pride. The duke/ex-fiancee was the last one to be cured because the black magic affected him the most. The other two was less because one was a paladin and the other was a dragon, but the duke was just a normal human. At the end, the paladin and dragon remains on good terms with mc but idk about the duke.
As for why mc transmigrated into the novel I'm not sure, but her body and soul had united, so her body adapts to her soul. Her hair eventually turned into a darker shade which will be kinda like a darkish/black purple (because she was korean so she had black hair) and her eyes changed into a different color. Her voice also changed to a lower tone, which gives off a calm and elegant voice. ML likes this change a lot because her hair color is now similar to his, and he likes the idea that everytime mc looks at her own hair color, it will remind her of him.