I believe that you can use the effect of reverse thrust when computing stopping distance on a WET runway. If the runway is dry, you cannot use the credit for reverse thrust. I know this applies for takeoff computations, and I believe it is the same for landing. As for single engine operations, there is no preflight requirement to figure one engine landing distance, it would have to be referenced from tables inflight or contact dispatch. Also, there is no preflight requirement to compute landing information for your departure airport.