Hmm, I doubt we'll get a full picture from Anamnesis Anyder. Maybe some more details on the tempering part, possibly some background on the creation process of Zodiark? One can only hope for some juicy bits.

As for the afterlife part, I always presumed that its akin to FFVII's lifestream. As in every soul comes from it, every soul returns to it. Souls returning disolve into it, and new souls are created from a random scoop when life is born. Echo wielders and ascians being the exception, as they have some means / strongly enough built soul to overcome the effect. Echo wielders would still lose most of their ego upon death, but preserving their... basic nature? I admit, this theory does have some holes in it, but not unmendable ones.

As such I think most if not all of the afterlife theories of heavens and such of the Twelve, are just myth.