I think this argument is fallacious because you're exaggerating how easily led Yuna is. If she had no strong convictions of her own, she would've just quit after Bevelle. Yet she didn't, despite the insistence of the two people you say she allows to influence and lead her along too easily. You can say Yuna's influence is more easily shown than other characters, although I would not be inclined to agree with that assessment nor would I say it somehow detracts from her character in any way even if it did.
You keep saying the evidence is "kinda there," but I've given counterexamples and you've been using mostly generalizations without any solid examples. Tidus obviously helped to change the way Yuna thinks, yes, but that doesn't mean she just lets him lead her along and she just thinks what she thinks he wants her to think. Yuna contradicts Tidus and Rikku often, and none of the other party members ever really try to tell her what to do.
Just pay attention to how the entire Zanarkand arc goes, and you can see that the decisions Yuna makes are her own. Yes, the influence of Tidus over the course of the game is clearly there (and it's not hard to see it in other party members like Lulu or Wakka either, actually), but that doesn't make her decisions less meaningful. That's the part of your argument I don't understand. So she wouldn't have been the same character had she never met Tidus and FFX probably plays out very differently for all involved if he is absent from the story, but so what?
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"I couldn't stand a day without a past or future. I need to live each moment to keep fighting here and now."