No. It's not energy-positive, fusion provides less powe than is put in. Instead it's a way to produce very energetic exhaust, resulting in better fuel economy.
No, this would be heating the propellant to higher temperatures than chemical reactions can get -- in principle one could get much more than 50% extra ISP for the same propellant mass. In rocketry/satellites/deep space propellant mass is a big deal -- anything you can do to reduce the amount you need without having to gain even more mass in other ways is a big win.
