I have another iterpretation of Balthazar. In my mind, after Isobel dies, Ketheric turns to Shar. He then builds that entire gaunltet of Shar to "raise" dark justiciars. Obviously, thjis is a super human endeavour. So, Shar herself must be involved. This is also the time that I think Aylin was captured (she was already in Eithrym before Isobel died) and imprisoned in Shar's abode in Shadowfell. It was Shar who directly or indirectly imprisoned her rival's offspring for eternal torment. (Why did Selune not interfere ? Selune doesn't seem to interfere with anything actually, e.g. Shadowheart's family, het temple falling apart etc...)
After the justiciars are defeated by the Harpers, Balthazar comes on the scene. To me, he is a Myrkul devout. He manages to revive Isobel and turn Ketheric from Shar to Myrkul. But this leaves Ketheric with a problem. If he wants to keep Aylin he needs to get her out of Shadowfell and in his domain. That is now Balthazar's primary mission, but he cannot possibly do this as he will never be able to pass the 3 trials. When the party arrives that's the reason why he accepts our help. Well, that's how the story goes in my mind at least.
That's pretty well thought-out, but we still run into the power-level plothole. Aylin's mechanical statistics and combat abilities are, relatively speaking, so puny that it wouldn't even behoof Shar to personally abduct her, a conjecture which is solely necessary under the pretense that Aylin is legitimately 'special' in some way. Tav can kill or capture Aylin, Lorroakan can do it, Balthazar and his undead minions could easily do it. Subduing some 80HP Deva with Smite wouldn't even require Shar batting her eyelashes, an act of bullying so unfair one might expect the Good-aligned Gods to be completely outraged by such blatant interventionism against an inferior creature ( Which is why even evil deities typically refrain from just terminating mortals nilly-willy ). If anything, she could just send Viconia and a couple of Sharrans to do it.
The reason you don't hear about direct divine children being snatched up by any aspiring arcanist on the Prime that often is because they're much, much stronger than Aylin. To play Devil's Devocate though, I can see a possibility where Selune deliberately nerfed Aylin's divine essence ( as a direct offspring of a Greater God ) to make her more humble or some such and then booted her to the Prime Material to preach the Selunite faith under the guise of a second-rate Celestial.
That is, if the game didn't repeatedly refer to her as 'Aasimar', which tells me that the whole 'Daughter of Selune' bit was probably just as hastily applied without much introspection for cool factor.
