Not sure what the point is here with all due respect. Mab still could have been Queen for a thousand years, still look young and pretty as a now mostly Fae. Who knows what child baring age is for a Fae? So Mozart or Schubert still could have sired her twins.
"Young and pretty" is irrelevant: I could ask in return what
your point is, there...? My "point" was that you had asked/speculated about whether Mab's twins were born as Lady or Queen.
From the metaphysics of the situation, it's very possible IMO that the "Lady"
cannot have sex & get pregnant. The "Queen" IMO could become pregnant whenever she wants -- as an immortal, she's unaging. The "child bearing age" I think begins as soon as she ascends from Lady, and ends when she ascends to the Crone.
I was addressing the historical question: Mab was Winterqueen for almost the entirety of the existence of "Austria:" there was no such named place, back when she was Winter Lady.