there are several explicit confirmations of a Sora and Kairi pairing within the game, which means it is, in that sense, substantiated within the source material.
According to your highly subjective, opinionated viewpoint on what constitutes a viable romantic pairing. There's nothing wrong with shipping Sora and Kairi if that's your thing, and you've really got nothing to worry about because it's probably going to be more or less confirmed by the end of KH3, but the problem is how you're framing the argument, or perhaps more accurately, the problem is how the argument for Sora and Kairi has to be framed based on the material as it is presented. To borrow a few of your examples:
1) Sora daydreams dancing with Kairi in Halloweentown. It's commonly referenced as proof positive that they're destined to be a couple, but this daydream only occurs from Sora's perspective, and can only be taken to mean that
he likes to think of Kairi that way. Kairi is not a participant in this fantasy, so while it can't be concluded that she definitely
doesn't reciprocate his feelings, this scene also can't be taken as evidence that she does. It takes two (or more) people to make a relationship work, and if you go through the material, Kairi is notably absent as a figure within this purported pairing.
2) Characters reference Sora's "feelings" for Kairi. This is an even worse example of how to establish or prove a pairing, because
neither Sora or Kairi's feelings are actually explored through this method, and Kairi is even further removed from the equation because nobody ever pauses to consider whether Kairi actually likes Sora back when making these statements. It's either assumed that she does, or assumed not to matter whether she does. Once again, to the extent that it can confirm anything, it provides an argument for Sora's part, but does nothing for Kairi's role in what is, again, supposed to be a romantic partnership.
2.5) On a personal note, I've always really disliked how the characters do this around Sora. Besides coming off as forced "friendly ahyuck" dialogue intended to shoehorn in romantic subtext to scenes where it's neither necessary nor natural to do so, it also brings up the discomfort of having grown up with people teasing me in the same way, making assumptions about who I liked based on my perceived gender and romantic-sexual orientation, and I have this fantasy where Sora turns on Donald and Goofy in a moment of totally liberated clarity and says, "You know what guys, my feelings for Kairi have never actually been stronger than my feelings for Riku or vice versa, I love them both and if I choose to date one or both of them that's going to be between me and them, and it won't be based on heternormative scripts as to what is and isn't acceptable or what is to be expected of my sexuality." And then Walt Disney's cryogenically frozen head will explode.
3) There's no canon material within the series that indicates that Kairi's feelings for Sora are more romantic than Riku's feelings for Sora or Sora's feelings for Riku. Kairi paints a reciprocal paopu fruit on the cave wall in DI? Sora and Riku get a paopu fruit themed Keyblade to share between them. Kairi rushes to hug Sora in TWTNW? Sora rushes to hug Riku immediately after he wakes up from diving into Sora's heart in DDD. You like the symbolism of Kairi and Sora having their hands pulled apart and then reunited as a bookend to the first two main games (I do too actually)? How about the symbolism of Riku reaching his hand out to Sora on DI in KH1 while Sora struggles furiously to reach him through the darkness, bookended by a reunited Sora grasping Riku's hand in TWTNW and crying as if he never wants to let it go? Sora is Riku's light, just like Kairi is Sora's, etc. etc. the list goes on.
The idea that fans can form an objective and canon reading of Sora and Kairi as
the couple for KH is entirely premised upon their heterosexuality as a justification because it's the only factor that separates them from Sora and Riku in terms of the probability of seeing their relationship confirmed in an explicitly romantic context. When I said that nobody would make the kinds of arguments you do if Riku was a girl, I wasn't putting words in your mouth, I was identifying a fact of reality: if Riku was a girl, the popular reading of the dynamic between Sora, Riku and Kairi would be that it is a love triangle, and Sora will have to pick one or the other at some point. It's disingenuous to claim otherwise.
I also believe that people of any gender or orientation can be friends without romantic strings attached. That's why I don't want any romance confirmed in KH: I want Sora and Kairi to remain friends with a powerful bond, and Riku and Sora to remain likewise. Sure, I would cheer if we got a canon, main queer couple in the games, because I'm starved for representation, but it's not actually my preference. I think Sora, Kairi and Riku all work best as a trio of friends with a single shared, special connection going back to their childhood: they all need and love each other, and everything comes from that spiritual center, the trinity that makes them whole. It's why I love that Kairi chases after Riku when he tries to leave her with Sora in KH2, and reconnects the two of them with their hands between hers (and subsequently why I hate that she gets left behind for the next 80 games). And why this is one of my favorite images and moments in the series:
Literally nothing matters to me as much as all three of them staying together, and if Nomura's really gonna pull some "people have to grow up and friendships have to change" in the middle of a Disney series about the magical power of being bonded with others inseparably (especially if that means singling Riku out as the lonely gay while every straight fantasy ever is reinforced) I'm gonna call BS on it. Because I don't like romance shoehorned into a story to the detriment of its actually valuable messaging, either.