No. Tidus is the centre of the stroy we see, but again, he is not the leader of the group.
The kidnapping in Luca had nothing to do with the end of the pilgrimmage. She kept going after that, and even after the Moonflow kidnapping, and only stopped once she found out that (SPOILER)one of her friends would have to be sacrificed along with her, in order to beat Sin the traditional way.
By the way, Yuna would definately have trusted Rikku... remember, they are cousins. In fact, I don't think the act that she helped Tidus earlier had any effect on the decision at all... all it did was convince Wakka (well, that and the fact that he didn't even know she was Al-Bhed..).
Oh, and... a character can have huge impacts on the story without being the 'leader'. Without Kimahri, for example, Yuna would have never reached Besaid, and the story would have been ALOT different, if una ever even did become a summoner. Does that make him leader? Without Auron, Tidus would never have even made it out of DZ, so he had a much larger effect on the story than Tidus did. So I fuess, by your logic, he'd be the leader? How about Wakka... he's one of the main reasons Tidus even became Yuna's guardian. If he'd listened to Lulu early on, things, again, would have been alot different. What I'm trying to say is... all of the main characters had a huge effect on the storyline. That's why they are main characters. Trying to argue that Tidus was the leader because he was important to the story is useless, because so was everyone else. Also, you were lead to believe that Tidus was alot more important than he actually was because he was the protagonist....