It can only lock (and maybe unlock the DtD) if it works in tandem with the RoL Kingdom Key on the other side of the door.
Let me rephrase: if it requires both keys to effectively lock the door, than that implies that unlocking it on just one side is effectively the same thing as unlocking it on both sides.
With my proposition, we could explore the deeper lore of these Keyblades rather than retreading the old plot threads.
I don't think anything you've suggested re: future worldbuilding options is necessarily improbable on its own face, but if Xehanort needed Mickey's Keyblade and had the capacity to steal it, he had a good opportunity to obtain it back during DDD, while Mickey was trapped in his fortress and Xemnas was choking him out. If Mickey losing the KKD is a plot point, and not just trailer misdirection, then him going to the realm of darkness just so Aqua can take it from him and give it to Xehanort is a very roundabout way of doing things. Perhaps Nomura is a very roundabout writer, but it still seems neater to me that if the KKD is specifically going to Aqua, it's should be because there's a specific value in Aqua being the one to have it.