I have heard people saying that the kh3 is a prequel but i don't think it is firstly when king mickey went on his mission in kh he didn't have the key blade at the time thats y he sent Donald and goofy to search for the key blade.
star seeker is a new key blade and in the other games he only uses his kingdom key so thats that.
i think the reason why mickey is the first on the scene is because that place is close to Disney castle, thats y sora goes there through a portal from the hall of the cornerstone
