We have no idea how long she was Lady, she may have ascended at the time of the Conqueror.
FWIW, we don't even know that she was ever the Lady! If both the prior Lady and the prior Queen died in the same battle, Mother Winter might have grabbed 2 unexpected choices -- even mortals! -- straight into those roles.
... the actual quote in Battle Ground Chapter 13 “This was never about raising an army, fool' Mavra hissed. 'It was about acquiring new blood for the stars and stones.” Add a comma after blood and the sentence changes completely, she is cursing ...
I don't think so. Add that comma, and the phrase "for the stars and stones" becomes more ambiguous, not clearly a curse (which would be "by the stars and stones" (akin to the curse "by God")).
But if this is a crucial hint/foreshadowing, I think Jim would have taken special care to get it right; no typos are likely.
... I still have no idea WTF Starborn means. Unfortunately I suspect neither does Jim.
I'm quite sure you're wrong on that last part. Our inability to be sure of the meaning (based almost-entirely upon the "unreliable narrator" elements -- and, largely, the complete ignorance -- of Harry himself) is intentional on Jim's part.
But Jim has been using "Starborn" and "Stars and Stones" for YEARS now; hse even had Eb point out that Harry himself had no idea (and therefore neither do we!) what "stars and stones" even means.
We know "Starborn" is a potential, shared by those (presumably those with wizard/magical power) born under a certain configuration of portents (mostly astrological; but perhaps not entirely?).
We know the Starborn have particular power over Outsiders, and resistance to Outsider powers; we surmise this is a
reaction to the Outsiders by Creation itself (or by the Almighty (or by one of his agents (
looking at you, Uriel!))).