1. Actually, I think the forest's aren't looking too bad at all. Could stand some improvement, but practice makes perfect.

2. I know what you mean. What I figured out had to happen was that you need to add a mask to every layer for the rivers once you've placed them. Initially, I had dirt going over my rivers too. What I did was I added the rivers as a black line over the white ground on the land mask, and reapplied this mask to all the layers.

3. Not sure if this will work, since I'm not sure if you can do a gradient from with transparent as a color, but try running a gradient from white (or whitish-blue) to transparent (if possible) on a separate layer above the land color layer (obviously with white at the north). If that fails, try running one from white (-ish blue) to 50% gray and set to Overlay (that should have roughly the same effect as the above). If all your bumpmaps are on separate gray-scale layers, this should preserve all the depth on the map. I'd try it out myself, but I don't have access to GIMP at work.