The mechanics of being a Fayth-Dream and/or Unsent are really up in the air, which is probably why they can handwave it as the reason Auron can travel to and from Dream Zanarkand. My issue is that it seems to exist entirely separate from Spira, though it can be travelled to; where, exactly, is the "entrance" per se?

On an entirely different note; do the Fayth dream the rest of Spira, too? What would happen if you tried to sail to the other locations in FFX from Dream Zanarkand? Tidus seems to think the entire world is a mystery to him, but he's really not a good example. The cross over with the Yevonite's prayer and it being the blitzball symbol for good luck makes me wonder if Dream Zanarkand was the Fayth preserving Zanarkand in its prime exactly as it was, or if there's some strange blurring effect.