she's not truly dense, she ignores it intentionally since she's using the novel a guide. and she can't really do anything spectacular since she was reincarnated as a commoner and ngl, the duke is not matured enough.
but she is kind of ignorant to the story , she should try her best as a duchess.
pretty bold to marry her knowing her background status or he's just a fool waiting to be tortured by blood thirsty men.