I'm gonna take this point by point too, but the first two flow well so I grouped them together. I don't think KH3 can be allowed to get away with saying the level design it has isn't better or worse, just "different", because the same can be said for every game in the series, yet we all agree that KH2 had bad level design. And as for airstep, Flow motion etc, it does impact platforming. If tools are given to the player to use for traversal, then the level design should work to incorporate all those elements. The only world that successfully did that was probably Olympus. Also no, I'm not just talking about the mirrors and I don't think they were "brain dead" easy. They did require you to engage with your environment. For the most part, KH3 level design is just KH2 but with wall running. As for the clunkyness, I can't exactly agree with that because it's not something I ever came across but I'll take your word for it, and also remind you that this is a much older build, so frankly I'm my eyes, everything it does that's on par or better than KH3 just makes it more so in my eyes.
If you don't refer to the mirrors puzzles in the World Within, then I don't understand what are you referring to, as the only other mechanics I can find in the worlds design is the clock and the torns, and very similar concepts can be found in many places in KH3.
Saying that KH3 is KH2 with wall running is not true, seems more like a biased view. You can actually complete the entirity of KH2 storyline with only a couple of mandatory jumps and never high jump because it's attached to a growth ability. I can think of
at least one moment or segment in practically every world that requires a lot more than simply running/wall running, including moments that requires air step, air slide, jump, sliding, underwater diving, flowmotion in other capacities...
I am not saying that 0.2 did not have amazing envioronments and good integration of gameplay and exploration. The same way, I am not saying that KH3 did an indisputable job with every desicion in their world design (i.e. Monstropolis is very linear and Corona is too flat for mi taste, and Arandelle is aesthetically too similar). I am saying that just because 0.2 did really clever design for some areas (the game gets flatter as you progress), we should not be unfair with the many, many good designs KH3 had, how different the worlds felt from one another, how big they are, how there are many things to discover if you explore, etc.
Much of KH3 cutscenes that aren't shot for shot remakes of disney films are actually pretty formulaic, and are simple shot reverse shot, with, again, Olympus being an exception. The Disney worlds in general aren't so plain, purely because there was probably a lot of collaboration between the original teams. Also notice, in KH3 when one person is speaking or in focus, nobody in the background does anything, not even an idle animation. This is not the case with a single cutscene in 0.2. Again, it's not impressive to me that KH3 sometimes matches 0.2 in this regard because again, it's a much older build, and should be the worser version.
I see your point in that in that a happens a lot of times in KH3, specially that weird scene at mysterious tower. However, there are many instances that it doesn't happen either, and just because the couple of scenes in 0.2 that are not Aqua alone or two people max handle the atmosphere and the interaction in a great way, again, let's not forget there are more than a couple of scenes in KH3 are good.
I'm gonna take these three at once, no, I don't think I can hand on heart say Kairi is anything resembling a character in KH3. She had a good first scene with Lea, but everything after that was so downhill that it soured me a lot on that one small, decent portrayal of her. She might have only had a few lines in 0.2, but she showed more emotion there than in KH3 (like when she expressed serious shock after hearing axel had a Keyblade) I'd rather have her in small quantities with her done right than large quantities where she's a non entity. Also she hardly speaks about Namine, certainly not enough to warrant it being a pivotal relationship for her in the game, although it should be.
While I do believe Kairi was underdeveloped in KH3, but she is still a character. A secondary character and part of Sora's character? Yes. But still a character on its own with some good interactions in this game. Again, a one time cutscene doesn't hold ground to put 0.2 as better or a diferent character than KH3 Kairi. I believe Kairi has been killed and killed again as character sometime during KH2 onwards, but I do feel that for every good moment she had in 0.2, she also had others in KH3 and Remind.
Sora has a couple good one liners, and the Disney characters have good meme lines too, but that's not what I mean. I'm not talking about witty one liners, I'm talking about characters seeing what happens around them, or pondering on what has happened to them, and coming up with their own views on the events, rather than "Well I don't care what's going on, you friendless loser!". I'm not saying an endless monologue in the style of Aqua is appropriate, but some introspection would be nice occasionally.
And yes, sora is a huge cut above how he was in DDD, but he's not really at the pinnacle of his character either, whereas in 0.2 Terra was given some time to really interact with Aqua after his possession, and he's much more apologetic as well as proactive on trying to lessen Xehanorts influence.
Wasn't talking about "witty one liners" or the memes. Sora, specially when talking with Donald and Goofy, show introspection in regards of feeling powerless, Roxas and Ventus. He has really good interactions with characters such as Woody and Hiro. He is affected by Hercules words after he leaves the world and remembers Roxas more than once during his journey after the events of TT. He is affected by Vanitas in monstropolis and he learns that Ven's heart is inside of him in that world. I don't know about you, but I think there are many many really good moments with Sora and SDG troughout the whole game, that in my opinion felt as good or better than Aqua's introspection.
Regarding Terra, again you are referring to a one-time scene... that actually is pretty similar to Terra's attitude in blank points and to Terra's return in KG in KH3. So no, while a good scene, I don't consider a point of character development for him, at least not good enough to be taken as an argument against KH3.
This has been proven by members of the community to not be the case, while there is still an element of randomness in both, 0.2 is much more consistent with getting situation commands, it's a fact at this point.
As you said, your points about the plot and disney worlds are more subjective so I won't address those, and I feel like I've already covered my opinions on the other points except for this:
Can you give a link to a comparison between the two systems? I'm serious, I've been deadly curious about how the system works.
The explanation I got in youtube from the Ultimania translations was that the meter fills up to a % of damage dealt with an attack (physical, fire, blizzard...). Once filled, the % of certain attack gives the probability to obtain grand magic and keyblade transformation. So if 40% of the damage was done with a fire spell, then the probability to obtain that specific grand magic is 40%, or something along those lines. I haven't found any piece of confirmation that the meter and situation commands work different between 0.2 and KH3, so if you have I would appreciate it very much.
I disagree, I think that this does make the platforming worse in KH3. Not to mention the total lack of puzzles across the board in KH3.
I agree with you that KH3 lacks a conventional type of platforming. In other games, you need to gain certain ability (like glide or double jump), to be able to reach an specific part of the level. In KH3, you pretty much have a full arsenal from the beggining that allows you to reach practically every part of the world. Refering back to the first point, KH3 has a lot of situations where you don'ts just walk/run, but actually need to do other things, such as air stepping, air sliding, double jumping, etc. Again, this is present in practically every single word in some fashion.
So at the end of the day it narrows to personal opinion. I understand why you, or other people, would prefer the approach of using a well timed jump to reach certain part of the level. For me, as someone pointed out previously, Aqua's movement in 0.2 is frustating in some parts. And for the size of the open areas in KH3, I loved air stepping and wall running, which in combination with regular jump and air slide allowed me to explore the worlds from the first time I landed there. And KH3 do have many things that comes from exploring using most of your abilities (I don't think glide is necessary in any point of the game), so it feels rewarding.