Sora has been shot at by guns in like, half of the games he's appeared in. He also holds guns in KH3 and Smash Ultimate. If Elastigirl can be cocking pistols then Sora can definitely slide with the holo twister assault rifle.
The majority of Sora's fans are adults, and Disney has allowed all of their other PG13 characters shoot guns... There's not really any guard rails around Sora other than maybe Nomura himself not wanting to see it
Which is why I mentioned an unrestricted context. Fake laser guns in a fake laser fight crafted by Nomura? Sure. A game shared with actual guns? I don't think so. Not because of a veto per se, but because KH isn't big enough for most companies to want to do the actual work to convince the relevant people.
This is not an absolute statement. Guns, real guns,
have appeared in KH. Notably, PotC with Will pointing a gun to his head (censored in the version Disney mostly cares about). But that was a retelling of a movie Disney made. And like you said, Smash, with characters like Snake. But again, that's far more contained. Sora isn't really getting headshotted by Snake, that game doesn't work like that.
I mentioned in my list that Disney has played with some properties (namely Pixar and some villains). I think that's partly Disney being laxer in general, and Disney wanting to do specific advertisements. I genuinely believe that Sora is considered closer to the Mickey side of the equation than the Incredibles side of the equation. Not in terms of importance (KH is barely acknowledged by Disney in the best of times), but in terms of what Disney will allow in terms of branding.
Just go look at the crossover wiki page for KH. You'll see a bunch of SE mobile games, Smash... and nothing. KH doesn't get crossovered. Occasionally a Disney show will have a cute one second nod, but that's it.
Like, genuinely, why would Sora, a character from a dormant IP that won't even acknowledge its own anniversary, appear in Fortnite? Who's doing that legwork? Who in Epic cares about Kingdom Hearts so much that they'll co-ordinate with the SE and Disney legal teams and do something it took Sakurai years to do, for a series played by (as you point out) aging millennials, and not actual Fortnite players?
Like, maybe tomorrow rolls around and I'll be proven wrong. But as someone who always wanted Sora in Opera Omnia (a full blown Square Enix game), I'd be extremely surprised.