1) C lunges up and hits with an overhand knock on Peter's head. First, is she ambidextrous? Because she would have thrown the knife with her dominant hand, leaving her off-hand for the rock.
She is using her off-hand with the rock. Desperate times do not make choosers.
2) Declare the prop. First of all, you said, 'knives' (multiple) in a description. Yet you only have C throw away a single weapon. We also need to know that C put the knife in her boot when she woke up at the beginning of the chapter. You also need to have a scenario where C shows Peter her knife and he knows about it. (At a table or to cut thread while she works with the women.) Otherwise, I have to suspect that he has seen her dress.
I have three, if not more, times mentioned that Catherine always have two daggers with her, one per boot. What I haven't shown is that others know this too. The question how Peter knows about her having A knife will be answered later.
3) For the fight...have C replicate a move that she saw while she watched the men fight during Andrew's training. Because she hasn't fought for this entire book, she hasn't worked out in secret...hasn't joined the men in the pell...I can see readers questioning her willingness to use violence.
I might have to clarify that her love/concern for Matthew override any sensitivities she has against using violence though. But I don't think she specifically needs to train for this moment ...
4) Suggest that you have C use the cloak to trap Peter's sword without allowing him to slice her. That way, the cloak issue is answered and becomes part of the fight. https://en.wikipedia.org/wiki/Cloak_and_dagger
Against Peter, a trained fighter/stronger man? She traps his sword, he punches her in the face. Game over?
5) Rather than having C hit him in the head from a kneeling position (and risk being cut), I suggest that you just have her break Peter's kneecap. That will stop him from following and allow her to escape.
A broken kneecap will not work for what I have in mind at the end ... I'd appreciate it if we could park this after you've gotten to the last posted chapter ...
6) Motivation for Peter...His hatred of Matthew should be foremost on the mind. Instead of taunting C about how she interferes with his plans, consider having Peter know about the tryst. Face it, the house steward knows. Therefore, he might have mentioned this to Peter. That makes killing C all the more sweeter because Matthew will be hurt by killing his lover. (If Peter knows about the knife in the boot, why doesn't he know about the affair?)
The servants and Walter's discretion is unquestionable, although there's a different way for Peter to know or at least suspect that much. He tried to kill Matthew once in Matthew's chamber when Matthew wasn't there ... And Matthew opened Catherine's chamber door to listen after he though he heard a noise ... Not being in Peter's POV means he has to talk about this. He's crazy enough though, so this is an option I have open, and will consider!
Thanks for this! Sorry about the absence, I was away for work and only came back late last night. Today so far was mostly a waste writing wise.