Okay, so, how do you know Dona had to backtrack?

I've always wondered why they need to get the aeons in a specific order. I mean, I guess you'd just have to end up heading back to Zanarkand, but still.

I've also always wondered this: they say Yuna isn't a full-fledged summoner until she gets all of the aeons. But as soon as she gets all of the aeons, she's gotta die. This doesn't make sense.