I'm not sure what you are trying to prove, but if you are trying to state that Sin can be chased away, yes, that is true. The whole point of the Crusaders was to chase away Sin (after they became the Crusaders).

It is evidenced that Summoners don't really fight Sin unless necessary, unless it is to help chase Sin away. Or that they are such a precious commodity that the people don't WANT them to fight until they use the Final summoning (Kudos to the Yevon coverup), even if they COULD damage Sin with normal Aeons (Yuna apparently can't says Seymour, but she fights Sin, 3 times before her final encounter? PLUS Jecht is Tidus' father)

Sin was always an Aeon, but after the first Sin, it's role and ease to control must have been much easier for Yu Yevon to exert. At first, Sin was the summon of MANY fayths, but after the first one died, it changed to ONE, and considering how much people hate Sin, it would not be completely moronic to imply that Yu Yevon took the Guardian's good intentions, and turned them into another monstrosity.

This also brings up what actually happens to your train of thought after you are dead... although this would probably draw in some aspects of being dead from actual religions. Basically, (albiet over simplified), after you are dead, you begin to learn everything and anything possible. This also means you see the "truth" of what you fight, but your zealotry overtakes you, and you become a mockery of that which you once believed in (even if through ignorance). This would explain Mika's behavior, Seymour's behavior AFTER his death (he was wasn't really different while alive, but he could not do anything about it), as well as other people's.
A nice analogy would be that you have a Republican, and he gets to become SO Republican, he turns out to support the Liberals, given the correct "information".

Overall, the idea is fundamentally sound, but doesn't really contradict anything that is debatable, but it does not add anything new. No offense intended.