I’m not sure I’ve gotten all of this sorted out in my head, so I’m going to lay it out here. Feel free to jump on and correct any inaccuracies.
Back in pre-history, The White God told all of the other gods to vamoose. Give up your immortality and live on, or keep it and have nothing more to do with mortals.
Someone, probably Hecate, maybe the Norns, instead of giving up their mortality, sponsored the fae. The fae, who had historically been the foot soldiers defending the Outer Gates, the keepers of the Outer Gates got a promotion. Their sponsor split their immortality into at least 8 mantles, 6 immortal, two not. Set up the two fae courts, one, Winter, to lead the defense of the Outer Gates, and one, Summer, to defend reality against the Winter court.
But, and this is important, The two tripartite divisions that are the fae queens, are still immortal. And The White God’s edict still holds true, their power was ordered to stop messing with humanity, so they cannot truck with mortals. In Summer Knight it’s described as fae queens cannot directly interfere with or kill anyone who isn’t attached to the courts through birthright or bargain. They they can do so indirectly with trickery, guile or glamor. Mortals, however, can still deal with them, since free will trumps everything. Free will allows mortals to deal with Outsiders, it certainly allows them to deal with fae.
It’s my theory, although not confirmed, that the fae queens must always be mortal when endowed with a mantle. Titania and Mab are sisters. Mab revealed to Harry that she was mortal once. Logically, that means so was Titania. At best they were changelings, but unless they’d chosen fae before they were endowed with their mantles, they would still be considered mortal. Aurora, Maeve and Sarissa were their daughters, arguably making them changelings, but if my theory that follows is correct, that’s not so cut and dried. Lily was a changeling, but she hadn’t selected her fae heritage yet when Aurora endowed her with the Summer Knight mantle and turned her into a statue. When Aurora died, She was endowed with the Summer Lady mantle, while still a mortal, albeit one wearing a mantle. Molly, of course, was fully mortal when she was endowed with the Winter Lady mantle.
The mantles that are not immortal, went to the knights, the hatchet men for the fae courts. These are mortals who the fae can sick on other mortals, to kill or hurt. Because the fae queens are constrained by The White God’s edict. Sure, the knights also will bring the pain to the fae, but what sets them apart from the other fae mantle wearers is their ability to kill mortals.
Run of the mill fae can interact with, kidnap, kill mortals. We saw that all the way back in Restoration of Faith with the troll. Jenny Greenteeth would have killed Billy and allowed Georgia to sleep on. Phobophages, Malks, Rawbones, Trolls, you name it, they have all threatened or outright killed mortals all the way through the books.
But the fae queens can’t.
In Summer Knight, Slate, in cahoots with Aurora, kills Ronald Rheul, the Summer Knight. No problem there, knights are supposed to kill mortals. Aurora buys it at the hands of Harry’s minions. Essentially her death is at Harry’s hands. He might have been doing the slicing and dicing with the bane, but with fae logic, he was responsible. The run of the mill fae that wielded the box cutters, again, had no issue killing her.
Loyd Slate’s betrayal of Mab is also revealed in Summer Knight. But, instead of killing him and allowing the Knight’s mantle to flow back to her, she takes him prisoner and tortures him for years.
In Changes, Mab doesn’t kill Loyd Slate, Harry does.
Fast forward to Cold Days. Harry believes that Mab is trying to kill him over 77 days. But most of those attempts are through predators, traps or fae. She only tries once with a pillow and once with a shotgun. Harry, pre-Winter Knight, could have protected himself from a shotgun.
Mab orders Harry to kill Maeve because Mab believes Maeve is Nfected. Mab had previously cured Leanansidhe of Nfection. So, why didn’t Mab just do the same thing to Maeve, her own daughter in order to spare her life?
Because, even if Maeve was wearing a fae queen mantle, she’s still mortal. The mantle makes her into a high sidhe. The mantle is what doesn’t allow her to lie, normally. The mantle gives her the vulnerability to the bane. Just as the mantle endows her with immortality. And Kringle tells us later that mantles can be worn, exchanged or discarded. Which would imply that in order to exchange or discard a mantle, you have to possess free will.
The Nfection counters the mantle and allows Maeve to lie. It also allows her to kill Lily. Maeve is, in turn, killed by Karin.
In Peace Talks and Battle Ground, the Fomor servitors don’t get the mortal protection because they are involved with the Fomor, who are involved with the fae courts. Molly, Maeve, Sarissa and Titania are allowed to kill them with impunity.
In The Good People, Mab encourages Molly to let her mortality die. Molly responds by getting in Mab's face. Instead of bouncing her head off a solid object as Mab has done with Harry in the past, she accedes to Molly's wishes and lets her be.
I can’t think of any time in any of the books or short stories where a fae queen has killed or injured a mortal that is not involved in some way with the fae courts, either directly or through the Unseelie Accords. I think there's a strong possibility that the fae queens, with the possible exception of the only original queen, Mother Winter, may all have been, and still are, mortal.
Comments? Corrections? Amplifications? Thoughts?