Responses
But what's to stop her dad from adding more and outright killing her with assassins instead? Her dad could just keep planting more and more people without the Duke knowing, people who could get through a security check easy or who have no problems with slowly poisoning her (which I assume is what happened in her last life). Her dad has a great amount of power, that's why her husband agreed to marry her. If he had been able to overpower her father at some point, it would have been okay, but the damage from years of neglect would have still been there.
When her dad threatened that he placed people by her, the obvious thing to do would be to just fire all the staff. It's not like she's particularly close to them. Or at least fire the ones that where hired around the time they got married and investigate the rest.