So... what exactly was the point of the intro story-wise? Why exactly did Leblanc steal Yuna's garment grid? Why did she put on a friggen concert as Yuna? What was she trying to do? I don't care if the intro was to "set the mood" of the game, this is the introduction to the story of the ENTIRE game and it's not explained why it happened in the first place!
Probably she wanted to piggyback on Yuna's fame, so she stole the grid to impersonate her.
Did you try playing the rest of the game? Doing something that's not very well thought-out but involves getting a lot of attention and adoration is absolutely and totally in character for Leblanc.
Hmm, why would a criminal group steal something to put on an event that thousands of people would pay to see?
It's spelled out later near the end of the game: Leblanc did it because she thought it would impress/draw the attention of Nooj. Presumably as part of this she would have admitted to him her role as the fake Yuna. Why she thought this would impress Nooj though is a mystery, unless it was supposed to just be the audacity of it, or because she was a former Yevonite (and had been slated to marry the son of New Yevon's leader) while Nooj led the faction which hated all things Yevon...
Why does the Celsius even need a Barkeep? There are only six people regularly on the ship (plus the occasional hitchhiker); can't they pour their own drinks?
Maybe he just makes a really good martini?
It's established that Yuna is really too nice for her own good. Perhaps he simply asked if he could join them and she couldn't say no?
But Brother (not Yuna) found him and let him on the airship...
Maybe Brother has some Qui-Gon qualities.
Why are there Fiends in the Farplane? Isn't that the point of a Sending, so that these spirits can be put to rest and not become Fiends?
Was it ever stated where the pyreflies of fiends you kill go? Maybe they flee to the farplane and merge into bigger fiends.
If Shuyin was able to corrupt Aeons, I don't think corrupting normal spirits into fiends is gonna be a problem...
If Vegnagun has been chilling under Bevelle since the Machina war and the Maesters knew about it then how come they never used it against Sin? It has the power to wreck the planet so why not aim it at Yu Yevon's space whale and permanently erase Zanarkand's memory?
Because they don't think Sin can be defeated.
Maechen mentions this in Chapter 5- the fact that Vegnagun can't detect friend from foe once activated made it too dangerous. Besides, even if they brought down the damn thing, as long as Yu Yevon escaped unharmed (which face it, is a pretty small target) he could eventually reform Sin.
It was a control thing. During the days of sin, Yevon controlled nearly all of Spira. People prayed to Yevon hoping to "atone" for their past sins in the hope that Sin would go away. They attempt to destroy Sin with a huge machina, it's a lose-lose situation for them. Sin is destroyed, they lose control of the people. Sin isn't destroyed, they're exposed as hypocrites for betraying their own teachings.
Shuyin thinks that he can end the Machina War by jacking Vegnagun and turning it on Bevelle. Thing is, he's absolutely right. Why did Lenne stop him? She's from Zanarkand too and as a summoner it's very likely that she's a part of Zanarkand's army so why does she care if her boyfriend annihilates them? Finish off Bevelle then fly back to Zanarkand and give Shuyin a Medal.
There's a difference between wanting to protect your home city and wanting to commit genocide. Lenne is not a sociopath and doesn't want to see any more bloodshed spent.
This. There's every indication that while Yuna and Lenne were spiritually almost identical, including being very gentle souls that don't even really want to see their enemies get hurt, Shuyin was a bit of a violent bastard and not a lot like Tidus at all. Tidus seems to be a heavily idealized version of Shuyin... and by "heavily idealized" I mean they seem to have wholly excised the part of his personality that would lead him to see "committing small-scale genocide" as an acceptable alternative to allowing his girlfriend to die, since I don't recall him ever trying to blow up part of Spira to save Yuna.
Also, Vegnagun has a mind of its own, and could not distinguish friend from foe, which is why it was never used. He may have only wanted to destroy Bevelle (at first), but it might not have stopped there.
That's why it wasn't used to level Zanarkand in the first place.
Why does Tidus revive now? The fayth trolled him two years ago about how he is going to disintegrate regardless of what happens. Now, the fayth shows up at the last second explaining he can revive him... To make it more ridiculous it's up to Yuna to decide and it would be completely selfish to say no.
Perhaps the Fayth had the power all along but they weren't allowed to use it. After all, if word got out then there would be an uproar of desperate people trying to have their loved ones brought back to life. Yuna saves the world twice and does it completely selflessly. The fayth figure she deserves a reward and ask her if she wants Tidus revived. Also bear in mind Shuyin corrupted the Fayth and had them attack innocent people. Perhaps they want to make it up to Yuna for forcing her to kill them again.
Or maybe Shuyin or Vegnagun's reassembling the Aeons gave the Fayth the idea/convinced them it was possible.
Okay, in the opening FMV, it's Leblanc and not Yuna performing the concert, because she stole the Songstress Dressphere, right? So where in creepy stalker lesbian Hell did Leblanc get a dressphere to imitate Yuna's summoner outfit, let alone her actual full appearance...?
She stole the whole Garment Grid. That's why she's able to impersonate Yuna. Presumably the Songstress Dressphere was the only one she had. Yuna was wearing her Summoner outfit when she first acquired the Grid (as the "Eternal Calm" prologue confirms). So when Leblanc put on the Grid with no Dressphere in it, that was the default outfit. She was spherechanging into the Songstress for the first time. I know the "Bare" Dresspheres would contradict this but that's only in the International and Last Mission editions.
Who thought it would be a good idea to create something like Vegnagun? Strong enough to destroy all of Spira in one go, artificially intelligent but not to the point where it's smart enough to tell friend from foe? Separately, why are its controls a piano?
The side opposing a group that thought "giant indiscriminate fishmonster holding our dead in purgatory" was a good idea - its pretty clear both sides of the war were both extremely desperate and completely, stupidly insane at that point regarding weapons development. The difference is, the ones who built Vegnagun were smart enough to realize that maybe they shouldn't activate such a poorly designed overpowered WMD, while the other side did activate their fishmonster and the rest is history.