The idea with that level was that Pete found a doorway to the past and reported it to Maleficent, Maleficent sent him through it to steal the cornerstone (which back then was sitting out in the open instead of being in the protection of the castle) then with the cornerstone gone she could send in heartless and such. Then Sora gets called in to fix it and you know the rest. What I find funny is that there is no butterfly effect in Kingdom Hearts. The only thing stealing the cornerstone did was remove the cornerstone from the castle. And in the process of getting it back encountering a past version of Pete changed absolutely nothing. It doesn't even seem to have changed the current Pete's memories (granted Pete in the past was just really confused by the ordeal) I have no idea what you were trying to get at with that one.