Yeah I don't think the discount applies to preorders. But they add towards the total. So making an order with a preorder and a game you want will get you the discount on the game you want (of course depending on the system and games you order). Then after it ships just cancel the preorder.
Not sure. I pre ordered it for PS3 since I don't have a 360 yet. Maybe when/if a 360 slim comes out.
You only get charged for what ships. Only SSFIV and 3DDGH were available so I only got charged for those: $29.99 (originally $39.99) for 3DDGH and $22.49 ($29.99 on sale) for SSFIV, plus tax...