I'm trying to figure out how you didn't get Auron's motivation or why he went about things the way he did.
He was right. The group did very much need to see the journey and the machinations behind it for themselves. These folks are devout in their beliefs. "Legendary guardian" or not, you expect them to take the word of some old guy that just shows up after a decade?
He couldn't have convinced them. Maybe Lulu. And even that is a huge stretch as she's rational, but still takes the whole affair very seriously and personally.
So yeah, I'm just surprised you arrived at "mean-spirited" for him. He's trying to avenge his friends, keep promises to both of them, and save his world. And he's already dead. He's borderline on par with Yuna for being the most selfless person in the game.
As I said, even if you believe they were so irrational that they wouldn't have taken his words remotely seriously, the group dynamics didn't justify the course. First of all, three out of the group of seven already had no allegiance to Yevon, so they could have been heightened in their awareness when dealing with them. Second, all he had to do was
tell them and then continue on the journey with them. It's called letting the mole work its magic... you plant an idea, and as the journey begins to reveal cracks in the facade, questions would have begun earlier. There was enough weirdness going on with the first Crusader attack and Yevon that questions would have started arising as early as then. And by the time they got to Guadosalam, after everything they'd seen, it is clear they probably could have avoided a lot of heartache with Yevon because even the most irrational believer would have been able to see something was wrong. This isn't like open-to-interpretation Biblical shit, this is seeing with their own eyes some seriously fucking broken and bullshit stuff Yevon was going on about.
We're treating these people like unthinking robots. Auron is treating his group of friends like complete morons. Maybe they were complete morons - there's an argument to be made for that. What would his friends who worshiped him have said to Auron's story in the first place? "We're sorry Auron we respect you so much and you're the only one of us who actually made it to the final aeon and your experience is easily beyond any of us, but you're a liar and fuck you!" No, the realistic course of action would have been they would have taken the story at least in the back of their mind and considered its content as they begun their journey.
But this is FF, I don't expect good writing. I still like FFX a ton