I don’t think it’s explained why people from Dream Zanarkand appear in the farplane. Tidus sees his mom in the farplane even though she’s not real (although she could be based on a real person, as FFX-2 explains Tidus is based on Shuyin). Jecht is also in the farplane in FFX-2 as he talks to you during the battle against Vegnagun. Whoever Jecht is based upon wouldn’t know about Yuna and so would have no reason to help her during the Vegnagun fight.
I’m not sure either. The people who appear in the Farplane would actually be the original. I’ve never woke up and had dreams based on real people stand side by side in real life. This is where gsurfer04’s theory comes into play:
I like to think that the main fayth in X weren’t “forced” to go to the Farplane after Yu Yevon died. There were summoners before Yu Yevon and nothing says the fayth are fundamentally linked to him. They went through their own free will and I think they could return if they really wanted to. To me, their appearance in X-2 supports this (though they were controlled in that case) and afterward decided to hang around a while to lend their power for Tidus so he can have a normal life.
How do these two theories connect? Simply, we can summon monsters in Final Fantasy X “into battle to fight alongside their summoner in place of the rest of the party for a period of time” and the same is for Tidus. In a sense, the Fayth summoned Tidus to fight beside (which is why, like the summons, you can name him).
But then why two sets of people in the Farplane? How can the Real and the Dream exist in the same place?
On a facetious note, it could be a rejection of St. Ambrose’s version of the “Ontological Argument”, where.
Anselm defined God as “that than which nothing greater can be thought”, and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God. He suggested that, if the greatest possible existing in the mind, it must also exist in reality. If it only exists in the mind, then an even greater being must be possible—one which exists both in the mind and in reality. Therefore, this greatest possible being must exist in reality.
If a Dream entity can exist, the Real version can also exist side by side. But I think not. If we can’t take this theory literally, we can’t salvage it.
Perhaps the Fayth helped them live a normal life, as avatars, in the real world. But I guess there isn’t a concept as true annihilation, and everything created stays in one form or another.
“The total amount of energy in a closed system cannot be created nor destroyed, though it can be changed from one form to another.”
Another question would be: how would Sin (who is not a Dream) destroy Dream Zanarkand? How can dreams fade away in real life? How can one be a dream?