So I was thinking, especially with all the Time Travel shenanigans in the Keyblade Graveyard, about how violating the established rules of time travel was possible, and I came to this conclusion. All of the realms in Kingdom Hearts exist on alternate dimensional planes. A key aspect to remember in this theory is that the higher your perceived dimensional plane, the easier it is to manipulate lower dimensions. A line, a 1 dimensional object can be moved in 2 dimensions. You can put 4 lines together to make a square in the 2nd dimension. In the third dimension, you can take squares, and make a cube. This manipulates the lines in a 3 dimensional space, which you could never do in the 2nd dimension.
We'll start off with the 3rd dimension, the Realm of Darkness. The 1st and 2nd Dimensions are Length and Width, imagine a line and a square. The 3rd is space, or depth. So a cube, space between 3 dimensional objects, all of that. In the Realm of Darkness, we see 3 dimensional objects, the space in between them, even creatures living and breathing. One key thing that we don't see however, is time. Aqua comments on this herself, with the clock tower in Cinderella's fallen world. Time doesn't exist in the Realm of Darkness. Which is why she's looking as fine after a decade there, as she did going in. This suggests that the Realm of Darkness exists in the 3rd dimension, but not the 4th. This could also provide an explanation for how Corridors of Darkness function. If they cut through The realm of Darkness, then they cut underneath time, allowing instantaneous travel from the perception of the Realm of Light.
The 4th Dimension would contain the Realm Between, the boundary between the Realm of Darkness and the Realm of Light. Time flows here as seen in Traverse Town, Twilight Town, Castle Oblivion, etc. This was most punctuated by Sora's year asleep in Twilight Town. Nobodies are formed in this realm, which is important because they're born lacking one thing we don't see until the Realm of Light. A heart.
The 5th Dimension and Beyond is theorized about, but we have no way of confirming what they are using modern science. So for this theory, the 5th Dimension, or the Realm of Light, is Heart. While nobodies can gain hearts, and heartless used to be hearts, it's important to note that they both originate from hearts, a heartless is formed from the heart of someone who gave themselves over to darkness, and the nobody from the empty shell left behind when a heart is lost. So these beings all originate from the Realm of Light, even though they primarily reside in other realms. You might think of hearts as more of a force than a dimension, and I would to, were it not for one thing. The Dive to the Heart. This seems to be a literal extradimensional space within a heart. The fact that it seems cut off from the rest of the world suggests that people themselves are 4th dimensional beings that contain a 5th dimensional heart, and thus live in a 5th dimensional world while perceiving it as a 4th dimensional one. Their existence in the 5th dimension allows the manipulation of time. Note the key rule of Kingdom Hearts Time Travel, you have to leave your body behind, only your heart can travel through time. The heart being the only 5th dimensional aspect of your being explains this restriction perfectly.
The 6th Dimension, as you may have guessed, is the Realm of Dreams. Time flows oddly here, different worlds flow at their own pace, and it's so relative and unchanging, that Sora and Riku, having travelled back a year to enter it, emerge a year later, having taken no more than days to weeks to complete their journey. This suggests it being 1 dimensional plane removed from the realm of light, allowing time to be manipulated more freely by outside forces. Dreams are made real in this dimension, and are observable and interactable with. Sora and Riku again perceive this only as a 4th dimensional space, being 4th dimensional beings themselves. But, Dream Eaters would be considered 6th Dimensional Beings.
The 7th Dimension, is the Realm of Death. Hearts usually don't make it this far. Although it wasn't given a name, it's implied that the essence that ends up here after death is Will, a 7th Dimensional aspect of people. This realm is stated to be the end, that there's nothing beyond it. I would suggest that Chirithy is mistaken, that there are at least 2 more Dimensional Planes.
The 8th Dimension, Kingdom Hearts. The entity that created all light in the world, that sparked life itself. The magical Big Bang of the Kingdom Hearts Universe. Since the Realm of Death isn't Kingdom Hearts, and Kingdom Hearts is shown to be all powerful, it must exist in an even higher Dimensional Plane. This Dimension is whatever makes the existence of life possible. But even more than that, The X-Blade implies yet another Dimensional Plane.
The 9th Dimension, A Dimension of Gods. The X-blade, serving as a Key to Kingdom Hearts implies that it and Kingdom Hearts were created by someone. Not something, not some force, an intelligent entity that created a lock and key for Kingdom Hearts. Locks don't occur in nature, they're an invention. This being, or race of beings exists on a high enough dimensional plane that they can not only spark life, but create an object that can spark life. This is similar to how you can't control time without access to the 5th Dimension. Or to reference Rick and Morty, "How you think you gonna move time while you're standing in it?" This means that this race exists in a higher dimensional plane than Kingdom Hearts. Which means that a being intentionally sent Kingdom Hearts down to the Realm of Light, to create the world. This being, is...
That spoiler was a joke, but it kind of fits. Getting back to the reason for this theory, Namine's interference in the Keyblade Graveyard. Because she was stationed in the 7th Dimension, and still had a connection through Sora's memories, she was able to manipulate time in a manner that no character in the Kingdom Hearts universe had been able to before. While Sora and his friends were unable to act differently, unable to change the past due to 5th Dimensional rules of Time Travel, Namine was able to Influence Lingering Will who is, as you're probably aware, devoid of heart and body. All that's left is his will, a 7th dimensional aspect of him. He was then able to act freely and rewrite time.
I don't know about the rest of you, but this theory explains it all for me. I don't think I've left a stone unturned here. What do you think?