Having another person with future knowledge makes things much more interesitng than having a single time-traveller. Having Cosette with the same knowledge would open so many possibilities for mind-games too.
To be fair, is Keira falls for such petty and childish schemes from the blong hussy, she deserved to be beaten.
Admittedly, she also ran out of luck with that dumb and ungrateful father who can't conduct a proper background check just because Cosette is a look-alike.
Maybe I'm dumb but I don't understand the premise of the story.
Even if she wasn't his biological daughter, it would be the mother's fault for committing adultery and his own for being deceived.
If anything they would deal with the issue in secrecy and not expose their shame in public like this.
In a few days the maids will just be :
Maid A : The Duke turned back into a human today as well !
Maid B : 'kay.
Maid C : Help us prepare breakfast already.