Considering Auron's dedication to his friends, I'd say if Yuna had decided to use the Final Aeon, he wouldn't have stopped her, because he swore to protect her no matter what.
Perhaps. On the other hand, he did desire revenge enough to try and kill Yunalesca once already and ended up with even more reason to want to kill her. I personally doubt he would sit and watch Yuna fall to the same fate as his two best friends. If this were true, he might have told Yuna of the Final Aeon before reaching Zanarkand.

And without the efforts of Tidus and Rikku, I doubt Yuna would've swayed from her path.
This is never implied. Yuna was always prepared to sacrafice herself for the calm but never, not once was it ever implied that she would sacrafice anyone else too.

She might've even married Seymour willingly.
This is about as silly as saying "Ba'Gamnan wouldn't have captured and held Penelo hostage if Vaan wasn't in FFXII". Side villians themselves are only there to keep interesting things happening when the main plot takes a break.

If you're gonna credit Vaan for bringing Ashe away from her misled path, give Tidus some credit for doing the same for Yuna.
I get that this was supposed to be his purpose but it's poorly executed. Tidus constantly tries to force on us that this is "his" story. Why are we reducing his role to influencing other characters if this is his story? The developers would have us believe that Tidus is the main character but you're now saying that he was there as an influence to Yuna. That would make him a side character at best.

And quite frankly, I doubt Rikku would've ever joined the pilgrimage had it not been for Tidus.
Again, you're saying Tidus is essential to the plot just because he fights in a random boss battle. Even if this wasn't a rather silly defense, i doubt the Al Bhed would have sent Rikku on her own and even if they did, she could have tried to flee rather than face it herself.

Really? You know, if you hate the laughing scene, fine, I can't change that. But at least accept that others may feel the same way about the whole Basch Fon Ronsenburg scenario.
I'm sure others will feel the same about the Basch Fon Ronsenburg scene as i feel about the laughing scene, but it doesn't change the fact that the laughing scene is far worse. Just type "Tidus" into google and "Tidus laugh" comes up in the list of popular searches. The laughing scene is infamous for its ability to grind at peoples sanity, the Basch Fon Ronsenburg is more of a minor annoyance at best.

I'll give you that Vaan has that one outburst, at least up until Pharos. And yet, I don't know what you're talking about with the whole Tidus keeps up his tantrum throwing. Please elaborate.
Vaan doesn't have an "outburst" in the Pharos. His second and last outburst was in the Bahamut and is smurfin awesome because he runs at Vayne and guts him with a sword ^_^

Tidus' outbursts are more child like. Example, when he sees Auron in Luca ("This is all your fault, everything, EVERYTHING!"), disobeying the temple precepts ("Like i care" - like a child disobeying his parents <_<), unable to comprehend simple things ("If you want everything you'll end up ith nothing", "But i WANT everything"), general outbursts ("I'm gunna KILL that Seymour"), and possibly the only one i'd say is justified (Lulu, how could you, HOW COULD YOU) but this one is waaaay too late in the game to still be having outbursts.

You can justify his outbursts and childlike behaviour but that won't make them any less annoying and it only makes it worse when only the one character is having outbursts while everyone keeps their cool. Tidus is a moron, he has the emotional level of a child and has trouble controlling himself. He's not a suitable protagonist for an RPG, he's barely a suitable protagonist for a saturday morning cartoon... or a pornographic movie. He belongs with the support cast and his doses should be minimal