So regarding Kethric's timeline, it's still a bit weird but here's what I think is tripping you up:
I think that Kethric didn't start using Aylin for immortality until after his resurection. Before that, he captured her in the Shadowfell and kept her imprisoned just out of spite, using the dark justiciar initiation as a way to torture her. Balthazar and the soul cage came into the picture when he was returned to life. The fact he had Aylin as a resource for the soul cage just turned out to be a bit of good fortune, after a fashion.
Okay this explains Aylin fully, but I'm still confused as to the timeline of Balthazar's involvement
Was it not Balthazar who betrayed Ketheric by striking a deal to summon Yurgir? Why would he do that 100 years ago if Myrkul was dead, only to turn to Myrkul about 80 years later when the opportunity arose? Aylin says she was trapped by Ketheric and Balthazar, but Balthazar is not a Sharran. Does she mean that Ketheric trapped her first in the Shadowfell, and Balthazar later with the Soul Cage? What was Balthazar's pre-Myrkul objective?
