I'm not knowlegeable enough yet to offer you any tips realated to your questions but I just wanted say that I really like your scene. I think it's a great concept. I've never used fog or done and clouds but the visual of your scene is eye catching for me. I just want to keep looking at it, hehe.
The only things I'm not very fond of are the two wood looking structures at the bottom left. My amature opinion is that they don't look realistic as the rest of your scene. Perhaps they stand out too much or maybe the shadow is too sharp, I don't know. Are those tunnels of some sort?
Don't be satisfied with what you can do but rather strive to do the things you can't do!
Exceed Expectations!