The game CLEARLY states several times that Kingdom Hearts is a place. You have to basically ignore what everyone else says to think otherwise. Nomura never said it wasn't a place either.

Quote Originally Posted by KH Wiki
Xemnas appears defeated, returning Sora to the Castle That Never Was, but Xemnas is still alive and well, basking inside Kingdom Hearts, urging it to join forces with him to gain the power needed to destroy their common enemies. Sora and the others try to stop him, but it is too late, and Xemnas is absorbed into Kingdom Hearts. Fortunately, the worlds created a door for Sora and the others to enter, for they wanted them to be the guardians of their destiny. They then follow Xemnas into Kingdom Hearts, but they see that Xemnas has taken its power and transformed the realm into a place resembling the World That Never Was with a giant fortress in the middle.
So yeah, KH is pretty much confirmed to be an actually place.

As for a heart being a place, KH is more than a single heart. Furthermore, it is not an unusual idea if you know anything about Buddhism. In Pure Land thought, Amida can be thought of being a mental state/state of consciousness, the nembutsu, his Western Pure Land, and some other things. This might seem tangential, but it purpose is to say that yeah KH is the integration of hearts but it can be a place at the same time.

Also, I assume that the In-Between Worlds are close enough to the Realm of Light that you can Gummi Ship it. TWTNW is closer to the Realm of Darkness thus the reason that the first time you go to it you must take a Corridors to Darkness/Dark Corridors.

Also, I was specifically talking about DTD and Dark Corridors in regards of travelling between the Realm of Light and the Realm of Darkness. I said "realms", I know but I wasn't really considering In-Between during that.

Wolf said that End of World is a place, not the DTD which is located therein.

I am pretty sure the two KHs never became one. There is a lot more evidence indicating two KH simultaneously existing than to theorize that the two became one. Basically, KH of Worlds is sealed away so Xemnas can't get to it, so he makes his own using the hearts freed by the Keyblade masters. I do not recall at any point there being indication for the seal locking away the KH of Worlds being broken or the two coming into one.