Don't make any theories about X until you have played X-2, because a lot of Spira's history is revealed in lots of the side quests and conversations.

Macalania forest gets it's mystical power from the proximity of Shiva. When Shiva leaves Macalania Temple at the end of X, the effects are much greater then at the other temples. The forest itself begins to slowly die, and the temple itself falls into the lake. Macalania can't sustain it's unique climate without Shiva to provide the cold energy.

Several places have mystical properties due to proximity to an Aeon. There are the flames of Kilika temple due to Ifrit, and the Electric Mushroom Rockat Djose because of Ixion. Macalania is no exception to the effects an aeon can have on a location.

Finally there is the mistake you are making that Yu Yevon summons dreams. He doesn't. Fayth summon dreams, and the only two people in present day Spira with the knowledge/ability to create them are himself and Yunalesca.