About Auron...

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(SPOILER)Auron went back to Yunalesca and asked for answers. She was outraged and attacked him, nearly killing him. As he made his way down the mountain, he told Kimahri about Yuna and somehow died going down the mountain. He was never sent, so he remained on Spira, and traveled back in time to Zanarkand through Jecht (who was/is Sin) to watch over Tidus until it was the time that Sin was to attack Zanarkand, and Tidus would really be sent into the present.

Massive Spoilers will ensueThere is no actual time travel in FFX. It just looks that way because Tidus is told that Zanarkand has been in ruins for 1000 years. Naturally he assumes that he got thrust 1000 years into the future. What really happened is that he was transported by Sin from Dream Zanarkand (DZ), which exists in some inacessable location on Spira to Baaj Temple in Spira proper.

So DZ and Zanarkand ruins are coexisting in time but not in space. DZ is fueled by the collective dreaming of the Fayth and will exist, along with all its inhabitants, for as long as they sleep and dream. Once they wake up, it vanishes, along with Tidus.

Auron in fact traveled to DZ only 10 years in the past (unsent and Fayth have no problem crossing over) to look after Tidus.

Mind you, this is refered to only obliquely. Some of this information comes from FFX Ultimania and not even the game itself. So it's one of the most common points of confusion.