It is confirmed by the Ultimania and there are clues in the game that Dream Zanarkand is an actual place. It is Sin's job directed by what's left of Yu Yevon to protect Dream Zanarkand from the rest of Spira. This is attained by having Sin destroy any place that is becoming to technologically advanced that could discover the whereabouts of Dream Zanarkand.
I wouldn't call Dream Zanarkand an aeon. Simply a recreation of the height of Zanarkand.
As for your theory regarding the "accidental plan", punctuation is greatly needed as it is a jumbled mess. But I understand what you are trying to convey. The reason Jecht has some control of Sin is because he hears the Hymn of the Fayth. That is how he is able to regain "consciousness" and control Sin. This cannot lay over with the other Final Aeons because the game does not state if there was some mechanism for them as well.
However I must say your "plan" has no weight. First how can there be a plan that Braska, Jecht? The plan arises from incidental events, not pre-meditated. If you recall in the game, Jecht gives up hope returning home so he offers to become Sin. That is not the group planning anything or obvious to any plan.
Jecht and Auron don't formulate any plan. Jecht asks Auron to watch over Tidus because he believes Auron can find a way to Jecht's Zanarkand (Dream Zanarkand). Auron is able to find Dream Zanarkand by riding Sin, the perks of being unsent, out to see and entering Dream Zanarkand. It isn't until the beginning of the game that any plan begins. As said by you, when Auron asks Jecht if he is sure; Jecht is wanting to bring Tidus to Spira in hopes that he could defeat him, end this existence, and probably end the cycle.
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