One Question That Needs Clearing Up...

[Masamune·1600]

If he doesn't bear to attack Dream Zanarkand, why will he go attack the Real Zanarkand?

Simply put, Sin never came into contact with it. Jecht disappeared at sea; given the danger to the spiral of death implicit in Sin reaching Dream Zanarkand, it seems likely that Yu Yevon would have done whatever he could to prevent such contact.

That's right.With Yu Yevon himself as the original Core, there isn't anyone to influence him or anything like Jecht for example, so he will have full control of it.

Obviously not. If Yu Yevon had full control, there would have been no need to establish Dream Zanarkand. Sin could have defeated Bevelle, and left the original Zanarkand to prosper. As repeatedly stated, Sin is intrinsically destructive, regardless of who or what forms the foundation.

This brings me to a question about what is Sins true nature. To bring death and destruction as Yevon and the Fayth claim, or did Yu Yevon have some other purpose in mind for Sin, eg only attack places where the people might become developed enough to destroy the Fayth or even find the physical manifestation of the Dream?

Sin's purpose was exactly as stated; it was armor that prevented any interruption to the Summoning of Dream Zanarkand. Whether or not Yu Yevon knew that Sin would and/or intended for Sin to be a force of destruction and bringer of death is a matter of speculation, until such time as official word on the subject is disseminated from SE. We do know that, so long as the tradition of the Final Summoning was perpetuated, the spiral of death could not end.

The people wouldn't become developed if the Yevon Church control them which is why Yunalesca strike a deal with them.

A culture's level of development has nothing to do with whether or not Sin attacks. The Yevonite Church controlled the people for its own purposes. As to Yunalesca, it is anachronistic to suggest that Yevon struck a deal with Yunalesca, as Yevon (the political entity, not the Summoner) did not exist yet.

Maechen:
"Rumors flew in Bevelle about Sin's sudden appearance."
"They said that the people of Zanarkand became the fayth, that they had called Sin."
"And that the man responsible..."
"was none other than the summoner Yevon, ruler of Zanarkand!"
"Yes, the lord father of Lady Yunalesca."
"On the eve of Zanarkand's destruction, Lady Yunalesca..."
"had fled to safety with her husband, Zaon."
"Later, the two used the Final Summoning to defeat Sin."
"Yet the people of Bevelle still feared Yu Yevon."
"It was to quell his wrath that they revered him, and first spread his teachings."
"And so were born the temples of Yevon."
"I suppose it's possible Yunalesca had planned it that way from the start!"
"A fair trade, she defeats Sin in exchange for her lord father's honor."
"Of course, there's no proof. No, the facts are lost in the mists of time."
"And who'd admit Yevon was an enemy of Bevelle?"
"You can bet the temples had a hand in covering that one up!"
"And that, as they say, is that."

I think people are looking into the story to hard, coming up with anwsers that arent really there. Just sit back and enjoy playing the game while trying to ignore inconsistencies and plot holes found throughout the game. But i guess if people enjoy posting and arguing go right ahead.

The answers are there, which I why I've been able to provide a coherent argument.

But since im here has anyone taken into account what the impact that Tidus's mother appearing in Spira's farplane might have for the previous theories.

No impact at all, really. Although the residents of Dream Zanarkand are manifested by the Dreaming of the Fayth, that doesn't reduce them to mere figments of the imagination. Rather, their "souls" are completely real, merely given substance by the Dreaming. However, as their physical link to life is through this Dreaming, its end means the physical death of Dream Zanarkand and its populace.

However, the residents of Dream Zanarkand are not immortal, and the population of the city remains in flux, like any human population. People (Dreams) die in the normal course of time, and go to the Farplane at death. The end of the Dreaming merely necessitates the entire city "dying" at once, as all lose their physical connection to Spira.