About the stitching, I used to do pretty much the same but without the crosses and multiply (which is a blend mode in Photoshop, not sure if GIMP has the same thing, but it means that light colors become transparent and dark become opaque). I found that it works fine to just match the edges of the drawing up, and soft-erase to hide the transition.

On to your map The shape of the continent is great, it looks natural and really cool. The detail of the coastline is too even, though; try to vary between detailed and smoother areas. One real-world example I've used before is the Mediterranean Sea.