You mean Lily? I remember a discussion not long ago. I'm not sure she really was the Knight. Imho the Mantle might have been temporarily stored in her body. Why didn't they let her remain the Knight? I can't remember exactly how it was.
Instead she became the Lady and Fix became the Knight.
Aurora got killed and than...? That was while Lily was still a statue? Where did the Lady Mantle go to in the meanwhile? The Unraveling turned Lily back. What happened to the Knight's Mantle? Didn't it go back to Mab or Titania? Did it do so, because the Queen(s) wanted it to or because Lily wasn't the right person anyway (i.e. female) and her turning to stone prevented the Mantle from leaving before? So Lily was free to receive the Lady Mantle.
Or did Mab actively call the Knight Mantle to her (or was it to Titania???) because they preferred a more suitable host regardless of the gender?
The Summer Knight died, and the mantle went to Aurora. Aurora made Lily the Summer Knight and turned her to stone. Aurora used the unraveling to turn Lily back in order to kill her. Harry killed Aurora first. The Summer Lady mantle went to Lily. Lily retained the Summer Knight mantle. She then chose Fix as her Knight.
The only part of that that is conjecture is that Lily was the Summer Knight, but until someone convinces me there is a reason to believe the could receive the mantle without either becoming the Summer Knight or already being a Summer Queen, I'm don't see a reason to entertain the idea.
Yes, I think there would have been war or other serious repercussions for Mab, that is why she wanted so badly for Harry to clear her of the crime.
What Mab wanted when the Mantle was missing and therefore there was an imbalance isn't evidence that Winter killing a Summer Knight would have serious repercussions for Mab.