The Triad, yes, but we have no idea when Odin was summoned. He might have had jack squat to do with the Allagans.
Wth regards to Odin: Sephirot's and Sophia's thralls needed to be killed like any Primal thralls. Presumably, without the thralls, once defeated, the Fiend and Goddess were gone for good. Odin seems capable of existing without any active worshippers besides the thrall on Sleipnir's back, and without any more belief than a vague rumor of a demonic horseman in the Shroud. Even when Odin is defeated and the thrall is killed...how does Odin survive until the opportunity to claim a new thrall presents itself? Does it have to do with the true form of Odin, the Zantetsuken blade, being an actual physical object? How does only one person's forced worship give Odin the immense power he has?