If you have access to DEMs then B is the way to go...just copy n paste and blur n paint until they fit seamlessly. Otherwise you're with the rest of us looking for a way to emulate a DEM...finding some techniques or doing it by hand. On the erosion, it shouldn't matter too much for a world map because you can only get so detailed in the image. By and large the rest of us just use informed guesswork and make it up as we go.