You can get 100% even if you do use cheats. I've beaten the game using codes 100% and 2 times without using codes 100%. I tested to see for different rewards but unfortunately you don't get any rewards for 100% on this one like you did in the previous ones. And to start a new game pause your game then go to the option 'game' or something and then under that select new game. Do you know how to load your game during gameplay? Like when you pause the game to load your previous save? Well go to that screen you'll find the 'New Game' option there.