My Hypothesis:
I would not consider Terra, Aqua, and Ven's Keyblades as well as the ones scattered across Sunset Horizon as false Keyblades. I would consider them as Keyblades that are created from other means, possibly from the wielders' hearts. So that would make them unorthodox Keyblades of some sort. What I believe is that Keyblades without Keychains are weapons derived from one's heart just taken form. If we think about it, its possible that Terra, Aqua, and Ven as well as those who wielded the Keyblades across the fields, did not reside in either the realm of light or the realm of darkness. Perhaps they had the potential to wield the Keyblades, but since they lived in a world inbetween, Keyblades unique to their own affiinities are created.
But we all know that the Keyblade can be be summoned, perhaps it could revert back as well. Well I guess we could assume it. Well if the Keyblades are connected to them, than it should repair itself after Ven awakes possibly. But then again, Terra, Aqua, and Ven's Keyblades could have started with them, so instead of dissapearing when dispersed, it would remain present, near its master, but it could revert back to normal... End of Hypothesis.
Normally true Keyblades have two constitutions, either from the realm of light or the realm of darkness. A Keyblade that acts from its own side is considered the exterior Key, while the interior Key can act from either side, but its powers does not change. But there is no such thing as one Keyblade wielder from each side. The Keyblades are spread across those who have the potential to wield them, so there could be infact many Keyblade wielders of the same realm. Mickey at first wielded a realm of light Keyblade, but in order to close the door to darkness, a polar opposite version of that Keyblade must be obtained. So Mickey left orders for Donald and Goofy to look for another wielder of the Keyblade because he would be off in the realm of darkness looking for a darkness realm Keyblade.
So their search for Sora began. With Sora wielding a realm of light Keyblade, he made his way to the heartless world. For the door to darkness to be closed, Mickey had to use a realm of darkness Key and Sora had to use his realm of light Keyblade to close the door on opposite sides. Two will close the door to darkness, a light warrior and a dark warrior. Nomura confirmed that Riku's Keyblade was no different from Sora's, he mentioned that its constitution was the same as Sora's, so its a realm of light Keyblade. Kairi's Keyblade is most likely is the same thing.
Hypothesis
We've noticed that Sora's Keyblade the Kingdom, Mickey's Keyblade the Inverted Key, Riku's Keyblade The Way to Dawn, and Kairi's Keyblade all have Keychains. This could mean they are connected to a particular realm. Also the UEM along side the DS, had Keyblades that had keychains attached, this could mean that the UEM could have wielded a Keyblade that originated from a particular realm. End of Hypothesis.