If Astarion were Cazador, then the PC automatically would become a vampire spawn, If they let Astarion feed.
Maiden Fel sounds like one of the sisters of Ethel, If you take the classical witch coven: Ethel is the crone, Fel the maiden and then there is the mother. Not sure If that make sense and what a hag would want with a vampire spawn, so probably just another crazy theory.
The theory of Maiden Fel wanting Astarion has to do with the infernal writing on his back. Someone on reddit deciphered the writing and it sounds like this "Hoyc inferiu non iurare per igneu, Naec virba loquor, Eoai mundo muoat." In English, that translates to "This soul swears no oath by fire, Nor words does he speak, In the realm of death." Some think that may be a piece to a ritual. This could be the reason why Maiden Fel wants Astarion, she told Galandrel that she wants Astarion unblemished. For what purpose we don't know.
