1. No idea. The last version I used didn't require it. I searched through the description page and every page of Nexus comments and it isn't explained anywhere.
2. You can, but it may not be easy. Or it may. It all depends how you configure the mod. You can make nGCD behave almost any way you want.
3. I have never had any problem using this mod and my own home-made race tweaks so I'm sure the same thing applies to RBP.
4. Tejon, I believe, wanted us to experience leveling the way we want to experience it. This is why he made his mod so configurable. We decide what kind of 'leveling experience' we want and tell the mod to provide it for us.
5. This one is simple. I think the D&D-style leveling system is asinine. One of the things I most like about the Elder Scrolls games is that they are skill-based games. Our characters get good at the skills they use. This is roleplaying to me. Accumulating "experience points" with, say, a Thief and then being able to put those points into magic-related or warrior-related Attributes, Traits or Feats is the exact opposite of roleplaying, as far as I'm concerned.
6. That used to be determined by other mods, such as Elys' Uncapper or AV Uncapper, not nGCD. You used to configure nGCD to allow those mod to take you past 100. How far past 100 was up to those other mods. Someone else will have to tell you if it is still this way.
::Edit:: I forgot to mention: I most often use http://www.nexusmods.com/oblivion/mods/13879/? It acheives most of the same things, is also highly configurable, and doesn't require a second mod to run.