I don't see a problem with Sin being brought back. That doesn't undermine the value what was done before (lives were saved, the temples toppled, etc.).
And as IMD said, Sin clearly won't be the primary threat. Someone else will either be exacerbating the problem with the Farplane or taking advantage of it. Sin will likely just be an occasional obstacle in the way of dealing with that threat, as well as a source of despair as the Eternal Calm crumbles.
For all we know, Tidus and Yuna getting along as soon as they are reunited could be considered normal in Spira's universe. If Wakka and Lulu could do it, why not Tidus and Yuna? XD
When were Wakka and Lulu separated?
Anyway, as you said, even in a fantasy title, "mild realism ... is the emotions of the characters." You can defy physics and we still buy into it, but you can't defy what we know to be human psychology and how our emotions work.
And Tresdias, in regards to the 'Bahamut created Tidus' thing, sorry for confusing you. We see throughout X that Bahamut very likely had a connection with him that was more special than any of the other fayth. In Dream Zanarkand, only Tidus could see him. The rest of the game, only Tidus and Yuna (and Auron) are 'allowed' to see his fayth form. (Of course, other summoners could as well, but Tidus was the only non-summoner that could at that point. Maybe the others could see Bahamut, but they didn't really address it.) He was the one who explained his and Dream Zanarkand's origins, and in X-2, you can ask him to bring Tidus back/initiate the Good Ending. Since the fayth are maintaining Dream Zanarkand, it could be possible that Tidus came from the portion of Bahamut's dreaming. The other fayth do have their own forms, which you can see when you revisit the temples after leaving Zanarkand. It's just Bahamut's fayth that appears to them for some reason.
But with X-2 and the whole 'Shuyin' thing... well, it is possible that Bahamut knew Shuyin and admired him. He and Lenne were popular icons in Zanarkand back then. Tidus is pretty much an imperfect copy of Shuyin's body with his own thoughts and memories.
Bahamut's fayth wasn't part of the dreaming used to summon Dream Zanarkand. He just provides the Bahamut aeon. The fayth can see into one another's dreams, though. That's how he and the fayth of Valefor, Ixion, etc. knew of Dream Zanarkand, why it existed, etc. According to the Ultimania Omega for FFX, the fayth of Yuna's aeons would go hang out around Dream Zanarkand. Things were fine and dandy as far as they were concerned until Jecht becoming Sin made them realize how **** up it all was.
As for Shuyin, Tidus isn't based on him. Remember, 1000 years have passed since Dream Zanarkand came into being. The resident based on Shuyin has been dead for a long time. Tidus is implied to be that reimagined Shuyin's descendant, however, in X-2's Ultimania Omega.
So while Nojima, could've had them become intimate in a more calm and romantic setting, having them become intimate in them middle of the storm serves a lot purposes...a reflection of their relationship, the degree of their intimacy (of course this is up to the readers interpretation), and it takes them to the unknown island to continue the story.
Insightful analysis. I really enjoyed reading it.
Honestly I am so disappointed in how this is turning out. I am hoping that this novella never gets a Western release and therefore can't be cannon (in our market anyways).
It won't get released outside Japan, because these things never do. That doesn't make them any less canon, though.