You don’t actually need to be on good terms with Lefebvre to capture her alive, I reported his actions to Hunter, so he pretty much hated me, but I still managed to convince him because my characters reputation was high enough.
As you silently creep into the dining room you come across the slim form of Lady Katarina at supper, her back conveniently towards you.
“Ah, Lord Aldershall. You’re late.” Katarina comments, not breaking her supper or for that matter even turning to face you.
Panicked, you attempt to fire off a shot but before you can even pull the trigger she jumps out of her seat and twists around to launch her plate at you in a single, fluid motion; sending your pistol flying out of your now stinging hand. A powerful boot sends her chair sliding towards you, causing you to trip and fall face first on the floor. Halfway through lifting yourself up a guardless sabre impales you in the chest.
“Did you really believe that would succeed, Sancroix?” comes the voice of Lady Katarina, from behind you. “Sir Caius, would you please dispose of this refuse?”
Your last sight before fainting is the confusing sight of Sir Caius removing his wig.
Really depends on how complex you want the whole assassination plan to be. You can’t really plan for your husband suddenly throwing you off a cliff and then blaming the wind and the wet ground or whatever. Doesn’t matter how much of a super awesome mega-great spy you are.
And yes, that is a pretty obvious assassination which would be incredibly suspicious and Caius would go and (try to) murder you without a question. But in theory it would have her killed, and no serious investigation should find proof that you are the guilty one (“Whatcha gonna do, find fingerprints?”). It would just ruin your reputation and your relationship with the people of Leoniscourt, and bring a murderous Caius on your heels. No biggie.
Then I realized it was a horrible idea. Also, killing her will do more harm than good, because you’re probably going to have all these distant cousins of the Cazarostas quarreling over who gets Leoniscourt, no one will approve of the King taking it himself, and it will just be a mess.
Plus, given that she is the Earl’s sole legitimate child, I can only assume she will be one of the most heavily guarded people in Tierra once her identity is made public.