Aurora wielded the Chlorofiend in Summer Knight in several blatant attempts to kill Harry. And the thing could harm Harry perfectly well, but it didn't seem to be able to hurt Murphy much at all.
While chapter 10 does imply that the Queens can only harm those in their Court, I got the impression that they could kill those in the other Court as well. I mean, Harry proved to be able to take out Aurora through treachery & deceit. A Summer & Winter Knight tends to be a smidgen more powerful that that.
If a Queen couldn't protect herself from a Knight of the opposing Court, you'd think the the lesser Queens would be switched a bit more often.
Then again, Aurora was slightly mad, so her madness might have allowed her to make an attempt on Harry (who was temporarily alligned with Winter). But not an unaffiliated mortal.
Also, in chapter 10 of Summer Knight, Bob states the open ended bargain makes him a target. Hence my earlier supposition, that bargains also let a Queen do things to you.