
What would be a wild plot twist is if the MC (or rather he body she is inhabiting) turns out to be the emperor's sister. She's confused when the emperor eventually comes along to kill Edward and all his people but sees her and flips shit. Then it becomes a deeper plot point and Edward further has a reason to marry her and become the emperor.
Not that it matters anyways (which is why this is a crack theory). Without a princess or any other family of the current emperor, he's next in line for the throne.

People keep calling the female lead creepy and saying that the author is failing to convey her cheerfulness. I think it's the opposite and that the author is purposefully failing to convey her cheerfulness and is instead making her force her smile to the point where it unnerves people.
She's a girl whose last memory was a horror show of her entire village being massacred by terrifying monsters. She wakes up to the boy she was going to marry 25 whole years in the future and he looks like a depressed old man. He tries to pretend he's someone else out of a clear sense of worry that he's now so much older than her when she used to be the older one that took care of him. Of course her first instinct would be to not want him to worry. Now she has to force a smile in front of the one person she can talk to about all this traumatic stuff as she bottles it up inside. Almost everyone she knew is dead and 25 years of her life doesn't exist.

What kind of mother does because she doesn't get enough nutrition because she doesn't like eating vegetables? Lol what the heck

That's what it said at first, that she lost her will to live because her workaholic husband was coming home so late and so often that he believed he was cheating on her and it broke her heart. Then it was saying that she died of lack of nutrition, not because she didn't feel like eating at all, but because she doesn't like vegetables to the point where they have to now hide it in her food like one would do for a child. Like what the heck

She says that maybe her mother's weak body is partly due to her lifestyle choices. So she's trying to get her eat healthy so she gets more energy and spends less time in bed. http://www.mangago.zone/read-manga/i_won_t_become_a_villainess_i_m_just_a_normal_duke_s_daughter/nbt/mpn_chapter-2196124/19/
It's not mentioned so far but that might again change the way she thinks since she's not lying alone in a bed most of the day thinking pessimistic thoughts.
I just think she's spoiled and will learn soon enough. Her family and the whole kingdom will know how much the MC did for them in the following months of her absence. Now that comment she made about not wanting to go to the wedding because the groom (who clearly did not care whom he would be marrying due to the exchange of letters even after his initial request) would allegedly ask to be her groom instead? That got me heated.