A weasel that keeps her word, which is more than you can say for Harry. Murphy tells him so.
He's broken
one promise. Admittedly, he broke that multiple times, but so far as I can remember he hasn't broken a promise since Grave Peril. These days, I would say that he's more a weasel in Mab's style.
To answer your second question In the relevant exchange Toot tells Harry it's Winter law and Harry says he didn't sign on, at which point the Mantle shows him the error of his way. While Harry may not know Winter Law, the Mantle does.
This doesn't answer my second question. Harry
doesn't know what Winter Law is. Sure, it sounds important, but for all he knows it is part of the bindings on Winter
faeries and it's part of his purpose as Winter Knight that he is
not bound by it. After all, Harry can still lie despite having signed on as Winter Knight and iron doesn't bother him unless he's stabbed with it, so there's no reason for him to assume that just because Toot subject to a restriction he would be as well.
Just because the Mantle is having a bad day it doesn't mean that it just quit working, it means it just quits working well.
...is there any evidence that the mantle
is working apart from Harry's back being broken? Because I thought all the evidence pointed to the opposite.
It turns out that the book doesn't support that position.
There's a WoJ somewhere saying that this point is deliberately unclear, and Harry doesn't know everything about what's going on. As such, I feel fairly confident that Harry contradicting my does not prove my position wrong.