If KH2 would be about reviving Maleficent and did the same thing she did in KH1, that would be repetitive. Lets not forget, DISNEY does NOT allow their characters Gravitate AWAY from their origins, so if Maleficent is an evil witch who wants to control the world, she'll stay that way regardless of Nomura's wishes.
They introduced new characters because they needed something original, if they continued the path that they took, and use the same Disney Villians over and over and over and over, then it will be KH1 over and over again, and nothing new will come out of it. The story I admit is confusing at times, but thats why there is we have these forums, so we can all speculate, rewind and review everything until we can understand the game.
If also like to point out you said in one of the FF forums that you have a very weak attention span which is probably why you found it a bit more confusing than any of us.