] Professor Randolph has also a pocket knife.
] Are the wounds of Mrs Parrish from the knife, which she was still holding in her own hand in the kitchen? I think not.

Our Caroline Bingley certainly would not do anything connected with a knife or a kitchen - in her right (used loosely) mind.

Today, DNA would determine Mr. Randall's possible association in all of this quickly, wouldn't it? But this is more fun.