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.