In what battles can you play as King Mickey in Kingdom Hearts 2

"You cannot play as Mickey or any other character(other than Sora)without using a cheat code." This isn't true at all. Ignoring the fact that the first person you play as is Roxas, and not Sora, you may indeed play as Mickey. To do it though, you must lose a boss battle. Actually, up to ten boss battles. In order, they are:
01-Shan-Yu
02-Thresholder and Possessor
03-Shadow Stalker/Dark Thorn
04-Cerberus (First Battle with Auron)
05-The Hydra
06-Volcanic Lord and Blizzard Lord
07-Storm Rider
08-Xaldin
09-Grim Reaper (First Battle)
10-Xemnas (First Armored Battle)
But there are certain limitations, such as not being able to finish a boss battle with Mickey, as he has no finisher, and that each time you die, you gain a lower chance of his appearing.