As calico stated - go through the tutorials of Oshyan. They explain the most important parts.
1. Those nodes were something I had troubles with, too. It is just a matter of training (learning to swim).
2. I assume that you mean surface shaders, as 0.9 did not use procedural terrains. If so ... yes

3. It should be possible - due to the size of a planet you will need a very (!) detailed Jpeg. Use the image map shader, which has different projection types.
4. A more complex matter, look at nvseal's planet-threads.
5. In the Atmosphere-tab you can change the density and many more parameters of the atmosphere.
6. This is explained in Oshyan's wonderful tutorials.
Volker