Do *not* use "a few" Compute Normal nodes! These are highly render intensive and generally you only need one or two, depending on the complexity of your node network and where you are doing your displacement. One may help your black artifacts problem, but it may also simply have to do with extreme displacement, or very small displacement scales.
As for the clouds, a distance shader can be setup to do this fairly easily, but perhaps even easier would be simply to paint out the area you don't want with a Painted Shader. Attach a Painted Shader to your Blend Shader input on the cloud Density Shader, check Invert, and then paint the parts you don't want out.
- Oshyan