1. There might be a rank above S. But you'll have to be a perfect and pretty fast minesweeper player if you want to achieve it.
2. There may be a way to fight the boss in free play in the future. (But for now, there is a super-secret keyboard shortcut that you can input on the map screen to get to the boss right away. Just press and hold a certain key on your keyboard for a certain number of seconds.)