These guys are really like actual scum. It's not like Grace wanted to be married to the duke or the duke wanted to be married to her. But of the two of them, the duke is obviously the one who is better able to deal with the fallout of breaking off the engagement. If these moron's plans actually worked and Grace was forced away (assuming she wasn't just actually murdered by the wild horse they gave her or by some random forest creature) isn't her life pretty much ruined forever and ever at that point? For the daughter of some no-name noble (who doesn't have any family support at all) to purposefully defy the highest authority in the land... isn't that just a death sentence?
So if these fucks are so determined to kill her they should stop being such pussies about it and just stab her instead of this wishy-washy bullshit they're pulling. And if they don't have the determination to kill her... then wtf are they doing??