Alright, first off, we know the rules of succession until we don't. There is a woj that basically is Jim stating it is that way until he writes it another way. So, there can be exceptions, peculiarities, extraordianary circumstances, etc that can alter what we know of the rules of succession.
For instance, the reason why Maeve thought she had won in part at Demonreach was that A) there was not an appropriate vessel for the Lady mantle that Mab was entirely sure of, B) candidates outside of circle on Demonreach were excluded due to nature of circle. Molly was a wild card that Maeve didn't foresee. So, her belief that it would just be safer for Mab to keep her than risk lady going to another present at Demonreach was in error, though it seems likely it was correct except for happenstance of Molly's presence. And, of course, Murphy not Mab killing her.
So, I could see a possibility in which Mab is confined within some impenetrable circle, and her death would release a mantle that would be contained within that circle. It is likely there are defense mechanisms for the mantle to escape but say some non-Lady character but an appropriate vessel was within that circle too. The mantle could pass to the non Lady making her Mab, but this succession would be outside of the succession rules that we have come to know.
Now, there probably would be a showdown between the Lady and the newly-mented Mab, but however brief her reign, the succession would have been altered contrary to the known rules.
As to Mab dying, first off we have multiple alternate realities and a likelihood at some point of different women holding the Mab mantle at a specific cross-time(infinity) event. Even when we have the same person as Mab, there is a likelihood that her different deals, and how she goes about either paying them off or collecting on them, contribute to differing modes of behavior across realities. Even say when we have Mab dying at point A in one reality, there is a likelihood in a high number realities that the point A event may not happen, or may happen slightly before or after specific point A time. Should there be a slight discrepancy in exactly when Mab dies between realities, and the resulting succession, then one will necessarily have a different person being Mab in one reality, whose reign begins slightly before or after, another original person Mab's reign ends.
Having stated the above, given the topic of this thread, and my favorite WAG, I propose that at some point in one reality Murphy is Mab. Harry doesn't find out til the end of this particular reality, when she essentially sacrifices herself to take out something greater. Harry (IdHarry) having glimpsed this in one reality wants a different ending for his Murphy and attempts a long game in getting our realities Harry and Murphy/Mab together more than how the other reality played out. So, aside from the Outsider, perhaps an end game of saving whatever is left of Murphy from the Mab mantle, having some love relationship between Harry and Murphy that another Harry missed out on....
And then, to include the other WAG, a little Marcone whose origin is tied into TT and unknown parents, namely Harry and Murphy who becomes at some point Mab.
Ok, pretty far out, but to add spice to the series after having read it so many times I like to read things into it and view the books through the lenses of some of these far out WAGs. Try it sometime if you like and/or get bored.
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