One thing that confuses me, is who was the fayth before her? She is found in a temple, so did the fayth there fade away? If that's the case why haven't the other fayths ever faded.
My view: Seymore's mom wanted him to use her as the final aeon, otherwise Yunalesca wouldn't haven't mentioned the mother-son relationship in the final aeon talk. But the whole power hungry thing came in before he was old enough/ready or whatever to fight sin. But i am probaly wrong.