Is Anima a Final Aeon?
Anima was indeed the final aeon. As for why Seymour didn't use it, well, Seymour started the pilgrimage when he was still very young with his mother and he was not aware of the final aeon and the final summoning back then. When his mother finally revealed her plan to become the final aeon so for the sake of Seymour, it left him devastated. He broke off the pilgrimage there and didn't proceed any further and thus didn't fight sin. This can be verified from the wiki as well.