There's also the Archive. Ivy is a human, with all the knowledge and power of a clearly supernatural heavyweight wizard, plus a lot more knowledge. She's also a Freeholding Lord under the Accords.
The Archive isn't a human with power - she's the human vessel of the Archive construct. If she was in control then there would times when she was more active, but the restrictions on the Archive restrict Ivy's actions.
Calling her human is like calling a Red Court Vampire human. Or any of the published vampire courts. Or saying a changeling is a human with fae powers. She has a human body, but there's something Other in there with her and that's enough to put her in another class of thing.
As for Kemmler...well...it wasn't just a matter of the Wardens, Senior Council, and a handful of others, it was everyone on the council who could be at all useful. And he was involved in flagrant violations of the Laws of Magic.
So what stopped him from going to Mab and demanding to be a Freehold Lord? If the bulk of the White Council was needed to take him down then had the power he needed to become a Freehold Lord and get protection from the White Council. I'm sure the Red Court, White Court, and at least one other "feed on the humans" group would have signed his application.
My view is that he couldn't do that because the Accords assigned him to the White Council's domain. I really can't see any Merlin to negotiating with Mab to be recognised as the chief of all the Human wizards. Maybe he had to give some minor points (like allow Trolls to eat naughty children when they cross bridges) but any wizard who clawed his way to the Merlin spot would have made sure that no outlaw wizard could claim to be a Freehold Lord...
But that's just my opinion and I could be wrong (see my speculation about the Shapeshifter). What I've typed seems to fit the setting, but in the next book Jim might change everything around.
Again.
Or in the book after that... More likely - he'll keep adjusting the world in all the books. And every time he changes something it will make sense because that's the way it always worked - we just didn't know it.
Richard