You forget Cat, this situation was actually done in game and it didn't work out. Her guardians are strong but he could have brought down armies on them. If they'd felt capable of taking on everyone to save Yuna they would have but there were held at gunpoint and helpless to save her leading to the blackmail....the guardians kill his guards while she dance-sends him. Durr. Problem solved. Plus, after the Macalania Temple, Seymour is still walking around alone a lot. Not always, but often. Plenty of opportunity to send him.
Also for moments when Yuna is alone with Seymour, let's not fool ourselves that dance takes time. Seymour could easily walk up to Yuna and whack her with his staff or something to stop her from sending.
HereYes, but that sacrifice, to her and the whole world, has always been "the Summoner dies" nothing there said a single thing about the guardian. Sure, the guardians may die during the pilgrimage, but nothing that their death was necessary to defeat Sin. And you saw how devastated she was when she found out about Meister Mika and her turmoil when she heard the Truth about the Final Aeon.
Yes, she was taught that her sacrifice is necessary to defeat Sin. Nothing taught her, until the end, that someone else's sacrifice was necessary. It shakes her entire view on how things worked in her world. Who's to say that she would have gone through with it? She wanted to ask Yunalesca something - that had nothing to do with Tidus - and when she found out the answer is "Sin can't be defeated, even if you use all your guardians as the Final Aeon" she decided to do something about it. She had already refused to do so, because she thought, "Well if my sacrifice and somebody else's really is necessary to defeat Sin forever, I might go for it... if it would be a pointless sacrifice of them... I won't."
Again, this had nothing to do with Tidus.
They didn't have a plan how to defeat Sin after defeating Yunalesca, I will not deny that. But rising against Yunalesca was not because of Tidus. It was because of the truth they found out and Yuna decided to not go through with it, and what Yuna says, goes.
It is Tidus who broaches the subject of asking Yunalesca for another way and help. Not Yuna. It's his speech that encourages Yuna to go along with it and find a new way. She only found out that Sin is TRULY eternal when they went to question Yunalesca, until that point Yuna like Braska still believed that with enough atonement Sin might not come back after it had been done.
Also of course she'd been taught someone else's sacrifice is necessary, it's the whole point of being a guardian to lay down your life for the summoner and ensure they fulfil their mission. She might not have known it was specifically required but she was certainly prepared for the eventuality that a Guardian would sacrifice themselves for the summoner.
If Spiran's atone then there's a chance that Sin might not come back. Yunalesca later reveals that humanity would never attain such purity. Until she does Yuna is still of a mindset that "There's a chance and it's not a pointless sacrifice".
