The flaw in your argument FE is that not once has any of the Ivalice titles have ever hinted that reincarnation is an actual part of the cosmology of its world. FFTactics even showed the land of the dead and it was very Mesopotamian/Shinto rather than Buddhist or Hindu.

We know their is an underworld, we know the spirits of the dead can be seen and return in places where the Mist is thick, we know they can influence the world of the living and if the theory of the Mist=Dark from VS is true, then we know the Mist has the ability to bring the dead back to life, though they are human no more. Actually reading through the beastiery, its proven the Mist can do this.

Yet no where is there mention of souls reincarnating. There is no strong evidence in XII to suggest your theory may be true.