Here is my thoughts regarding Mickey's Role:
During the Events of Birth By Sleep, Mickey most certainly was not a King. In my opinion he was in training to become a powerful knight, which would lead him to rule a Kingdom in the long run. So its likely that he's an apprentice and was trained using a blade and how to manipulate spells. In my opinion around this time something caused the worlds to shake up a little (possibly due to the creatures and the vanishing of Master Xehanort and his apprentice). I believe Mickey was a chosen wielder of the Keyblade and received the Star Seeker (a realm of light Keyblade) to fight off the threat.
The young Mickey shown his new weapon to his Master Yensid and he told him of the Keyblade legend and how the world was in trouble. I believe around that time Mickey could have completed his training under Master Yensid, Mickey could have been sent out to find others who wield the Keyblade. As young Mickey would go off and to find others who wield the Keyblade, he would be considered a Master over the trio, Terra, Aqua, and Ven, who all would take orders from him, as he was an elder and a more experienced knight who knew of the Keyblade's legend and history.
That's my thoughts.
