Between "fate is unable to be changed and you just have to accept that" and "fate can be changed," which message does KH3 best support overall? Both concepts are "supported" by characters who also contradict themselves: Sora claims a "real leader" needs to accept things are out of his control, after actively rewriting reality via the power of waking, while Xehanort wants control, despite his DDD talk of inevitable fate. Does KH3's story support either more than the other?

I don't really think that's what Sora was trying to express, personally. I don't think he's so much as saying that individual people shouldn't try and resist destiny, but rather that destiny is not (nor should it be) something that can be wielded and controlled like a weapon. That's what Xehanort wants. He wants to be Destiny, to control and decide how every single person lives and what their outcomes are. Like, he's basically going on about how he thinks the only way to save the world is through fascism and dictatorship and Sora is saying that someone with a truly strong heart, a real leader, would not try to dictate the actions of others, and, if there really is a need to save the world like MX suggests, the answer is not in one person deciding everything for everyone.

Especially when you consider how much MX attributes 'fate' to situations he intentionally orchestrated. Throughout the series he's rhetorically used destiny as a weapon, (claiming every awful thing he's ever done was just 'putting them in their proper fated places') and his goal was to literalize that, to have the power to cosmically do that for every single person. To me that scene just is not at all actually about whether or not you should accept bad things happening to you/fate/etc. It was about standing up to fascist ideology

As for what KH3 supports in general in that way, I think it unequivocally is in support of individuals trying to defy what seems fated, and I don't think there's any real doubt about that. When you open a game saying 'the destruction of everything will happen and there's no changing it' and another character saying 'but what if i can' and then that so-called fate is thwarted, it's kinda hard to see how the game is saying anything else. That isn't even getting into the more character-focused story of just trying to save everyone from their so-called 'fates' at Xehanort's hand, which obviously by succeeding is further saying something to that point.

But honestly, destiny/free will/etc is the least interesting axis on which to view KH3. It is much more a story about moving on after and recovering from trauma through compassion, empathy, and connection with others. Even Sora's more personal story in the game, honestly, has nothing to do with destiny.

Sora didn't know about the prophecy. He was not actively trying to 'change fate and save the world' during the tear in time, he was just trying to save the people he loved when it seemed most hopeless. Like, I don't really think that's actually about capital-d Destiny, I think it's trying to thematically express something more human than that, especially when considering that Sora is a stand-in for the player.