It's been a while since I've used gimp so I don't remember exactly how, but there should be a way to make a brush with a lower 'hardness' value. Lowering it should make the edges of the brush strokes less sharp. Basically the brush will get progressively more transparent toward the edges.

As for the switchbacks: I think what you want to try to do is gradually shade the bottom edge and corners of the road. Currently they're just floating in space on top of the shadows rather than casting their own.

I hope that helps.