Currently niggling at an idea that is barely more than idle speculation, so please bare with me as I air my thoughts.
I have some Tom Holt and Wizard of Oz movie mixed up in my head.
What happens if certain non-Hermetic magic users/witches really are another species? Witch covens are organised by troll witches to prepare converts into becoming more troll wives? Say a special ritual to transform the prepared coven members into green skinned, ageless witches, but only during a lunar eclipse when the correct conditions occur, say once every few centuries.
Gruagachan transform into trolls due to Final Twilight. Bjornaer into Great Beasts if they are prepared enough.
Lots of other various attempts by Magi to transform into immortal before riddled with Twilight.
So why not a non-Hermetic route?