Yes there is. A good rule of thumb when judging whether or not something is canon is by asking whether or not it contradicts canon. And trust me, some of these fights contradict canon.
They contradict what you believe about the story. Even the Lingering Sentiment fight in KHII was eventually proven canon.
You mean MF? No he didn't. Not the fight anyway. Only the character is canon.
The fight cannot be canon.
It takes place in destroyed Land of Departure.
Neither Ven nor Terra could have fought him, knowing that much (Ven never went to LoD after it was destroyed, and Terra was only in it momentarily, having gone to KG right after). And the timing is impossible even for Aqua. The only time she was in the destroyed Land of Departure was when she brought Ven. But it's not like she dumped his body and went exploring, she immediately went inside the castle and turned it into CO.
So, the fight just didn't happen. It's not canon.
If the Mysterious Figure appears before the final battle at KG (as it does according to the completely unlocked gameplay), then it's just as canon as any other optional world or battle in the games (for example, Atlantica in KH1). Or, as an almost exact parallel, Xemnas in KH1. If none of the battles were canon, as you claim, then the character quite simply wouldn't be canon in KH1.
From the cutscenes presented (ignoring the unlocking mechanic, because once the battles are unlocked, the game presents it as happening before the final battle with MX and Vanitas), we have Terra, Ventus, and Aqua returning to the Land of Departure some time after Master Xehanort destroyed it, and some guy immediately shows up. Now, to get the treasures there, each of them have to have returned there anyway, and Ventus himself can only go there after the world is destroyed.
Obviously the Ultima Canon is... canon. But how he got it isn't, that's just a gameplay gimmick.
The Mirage Arena is not canon. If it was, well, good luck trying to explain how it exists. More importantly, with new bosses like Monstro, Eraqus' LS, and No Heart, it literally makes no sense that they're in the Mirage Arena plot wise. How did the Mirage Arena retrieve data on them?
Given that the Mirage Arena is the medium for online multiplay, you should absolutely not regard it as canon. It'd be like saying that the online for Call of Duty is a part of the campaign.
The problem is, there's nothing in the games that says it's NOT canon, and stuff like it (Sora beating the Hades Cup) was proven in KHII and Days as canon. And the fact that Terra obtains the Ultima Cannon according to canon, something that can only be obtained by defeating the Iron Imprisoner, seems like an almost immediate assumption that it's canon.
People claimed that the Unknown in KHFM wasn't canon, or that the Lingering Sentiment, Kurt Zisa, Sephiroth, Ice Titan, Hades Cup, etc. weren't canon. Later games and Nomura himself have consistently refuted those claims.
You're idea that the Mirage Arena is drawing from them as data would actually explain the Mirage Arena quite well, and fits in totally with the aesthetic of the Mirage Arena. It might not be how Nomura eventually decides to explain it (if he ever does), but it shows that there is absolutely nothing keeping the events of the Mirage Arena from being canon. It would even allow a perfect method for multiplayer, without even resorting to Pokemon's canon explanation of communication with parallel universes.
The journal isn't what dictates the canon-status of something. It's meant to be a record more for the player than it is for the character.
The journal, and cutscenes, are explicitly presented as "story". While they seem much more player-determined, they are still presented as story. Either that story is canon, or it isn't.
No, just because it appears there doesn't define it as canon, if you disallow the game itself from demonstrating canon. However, neither later games nor Nomura have ever reduced the cutscenes to non-canon status; the best you get are retcons (due to FM's and Re:'s) and some but not all of the ancillary material (the secret videos and manga).
No, even if it has cutscenes, it might not be canon. For instance, the MF fight.
This is a circular argument. Sephiroth in KHFM was proven as canon by the Ansem's Report he drops, and the One-Winged Angel memory in CoM. The Unknown in KHFM, a situation almost exactly the same as the MF fight, was proven as canon by KHII's own intro.
Additionally, the data battles.
It is pretty much impossible for Sora to have fought Xigbar, Roxas, Saix, Luxord, and Xemnas in data battles because the real fights all occurred in the final world. People argue that Sora might have stopped between boss fights and gone back to Radiant Garden, but that's ludicrous. The narrative makes it pretty clear that this was all done in one trip. Sora was going to save Kairi and stop the Organization for good, I don't think he would just happen to decide to go back to Radiant Garden.
Even if that doesn't win you over, we know for a fact that a data battle with final form Xemnas is literally 100% impossible.
If it's data, as the game plainly says? There's tons of possible ways for it to make sense.
#They did stop and go back to Radiant Garden, to recuperate, just as they did in KH1 against Ansem (both to Traverse Town AND Hollow Bastion).
#He didn't need to fight them first in canon, and the unlocking mechanic (as with the Unknown, Vanitas Remnant, MF, and Lingering Sentiment) merely changed the game from a locked state to representing canon as it truly was.
The narrative makes it pretty clear that this was all done in one trip.
It absolutely does not. It plainly depicts them taking breaks to just rest frequently, not at all like Roxas, who was shown (and forced through gameplay) to storm the castle. There were plenty of opportunities for them to leave the castle, and have it still fit into canon.
On an even stranger note, Sora must have synthesized the Ultima Weapon in KH, and obtained the Metal Chocobo, Lionheart, and even the White Trinity Lady Luck, because they are all memories of his in Chain of Memories and Re:coded. He even had to have removed Oblivion from the chest at Hollow Bastion, because it symbolizes his memories of Riku as shown through Roxas, and used Mushu as a summon, because Mushu recognizes him in KHII.
As an overarching view, the principled claims are "only what you must do to beat the game is canon", "anything with cutscenes is canon", "anything at all in the game is canon, including gameplay". The first is plainly disproven, since entire worlds can be skipped in KH1, KH2, and possibly KHD (I haven't checked), but the games set after them claim that those events went through. For the second, while Nomura has never explicitly said so, the "future games" and Nomura's statements have consistently backed it up, even approaching #3, with stuff like Mushu, the memories of Keyblades, or the Anti Form. You even have stuff like Merlin in KHBbS saying that, somehow, the Command Board is canon (whether it is in their heads or not is a different matter).
Yes, with all of this being canon, there are many odd situations in which Sora takes a break from throwing himself at stronger and stronger enemies to go train himself or find better weapons....so?
Isn't that what you would do? If you did it, why is it so ludicrous for Sora to have actually done it?
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