I find Yuna's naïvité and Auron's general secrecy to kinda go well together because they both center around the same thing - which is that even if you get told something, you don't understand it until you experience it yourself. Even if Yuna hears Jyscal confessing Seymour killing him, she doesn't process what that means about Seymour, it goes so against both everything in how she herself lived and even the values she made herself be an embodiment of. It's IMO tragically quite realistic how much of a mental block and stubbornness she has towards facing Yevon being filled with not just one but several maesters who have nobody's goodwill at heart and have zero intention of seeing justice done, it's completely counter to everything she believed in.

Auron understands this kind of thing very well, which is why he chooses what he says outright very carefully and more than anything just lets them experience what he knows awaits them on their own terms and drawing conclusions themselves based on that. He himself also knows Braska will eventually die but never processed how much that actually happening bothers him until the eleventh hour, either. The one time he kind of oversteps is at the bottom of Macalania Lake where he's already saying "we'll defy Yevon if we must" but the party is still far behind processing that this is where things are headed and dial it back several gears. And lo and behold, they experience it directly and are led to the same conclusions, but now they actually understand why they must.

I love this kind of thing personally, I think it's all written in a really believable way. Though overall I do think the whole Bevelle section is somewhat contrived on an event-to-event level, and Seymour is the one part of the game's story I actively dislike, with him only getting worse past Bevelle as well. When your own party's response to every one of his frothing rants is "man this guy's smurfing crazy" you should know you haven't written a compelling villain with interesting motivations providing a different perspective.