I generally agree, that Kairi was handled poorly in this game. In hindsight it's clear that this game isn't anywhere near a Kairi game, if anything it's pretty apparent that the decision to include her was made in the final hour of the production.
Still with how they hinted at the game and advertised it, it's not too surprising that people got the wrong impression. They showcased her prominently as the hook in the first few trailers and placed her front and center on the cover, which in the very least should indicate a bigger role. Even Interviewers got the impression that this was a Kairi game and SE never felt the need to correct them, other than the one time Nomura said he didn't think of it as a Kairi game.
Still the choices they made are questionable. They made a model of her, yet only used it for a short segment, seems like a waste of resources. They had her replaced by Sora, for the final battle. Which again seems like a waste of resources, seeing the made a whole new playable model for one single fight for a pre-existing character.
Also it does seem weird that they then didn't include those models in the final credits.
I'm not too thrilled by how they had her fight with Memory-Nort play out, cuz this basically states, that Sora carried the fight against Xehanort all by himself in KH3. She couldn't even land a single blow, which means she probably didn't contribute anything of value. Which doesn't just devalue Kairi's contributions to the fight, but also somewhat cheapens Donald and Goofy contributions to the fight against him.
Her not even being able to defeat her memory of Xehanort, was an interesting choice, seeing this is the last chance for her to overcome her tormentor, kidnapper and murderer. While I agree she couldn't and shouldn't be able to take out the real Master Xehanort on her own, her memory of him should have been fair game. Or they could have always had her fight Apprentice Terranort instead.
While I can somewhat understand the choice to say she's not ready to go to the other realm, I also find it odd. Sora, Riku and Roxas also didn't require any training or preparations to go on their adventures. And they really have no reason to assume that this new Reality would be dangerous, it's unknown, but so were all the other worlds they visited on their journeys. Plus out of game we all know there'll be a level reset, so the point will be kinda mute.
I'd like to think that Kairi training with Aqua, will yield interesting results. It'd be fun to see those two interact and see what kind of Master Aqua is and how she'd train Kairi. Sadly I think we'll see about as much of it as we did of her Training along Axel.
However in the end we all know this training just a benching and that, the promise of her fighting side by side with Sora and Riku is a hollow one.