Lads... Yet another Angela theory (I'm a little bit sorry I promise)
(just a heads up, very light spoiler for the fork, the witch, and the worm)
I'm sure this has already been suggested since I fear everything has been suggested at this point, but could Angela be another victim of a blessing gone awry like Elva is.
It could involve the interference of a bonded dragon as with Elva, it could be a wild dragon - before or after the formation of the riders - it could be a member of the grey folk, it could be some other magic capable individual or member of a race known or not.
It could also be an intentional curse, perhaps by Tenga himself. Maybe he was victim to a blessing gone wrong and sought to create someone like himself intentionally or is part of a long line of cursed individuals (something like this is my current head canon).
I'm sure that someone more knowledgeable than me could cite exact textual references to support this but I think the most general evidence is Angela taking Elva under her wing as her personal student. (As for my personal head canon, I'd like to think that her finding out that he did this to her on purpose is what led to their falling out)
Anyway, I now await ibid to quote Mr Paolini disputing this when someone else suggested it as a theory.