If Zariel was an angel that fell and became a devil, I'd find it very odd to believe that devils couldn't rise to become angels.
Same thing with the Fey Llolth becoming a demon. That can happen, but not in the opposite direction? Doesn't make a lot of sense to me.
Perfection marred is eternally imperfect?
Eternal damnation is... eternal?
Doesn't sound all that all-loving and forgiving, but, feels like it has precedent.