You said you found the knife in Chitose's room right?

What if the culprit only planned on one murder? So they left behind the knife when they decided to pose her.

But Miki came out at some point and found them. And she came out with her shovel, since it sounds like it was important to her?

Since Miki saw everything, and... [a pause] would have known the killer, they had to adjust last minute.

She was pretty good at defending herself, thanks to what her home was like.