I'm certainly not an expert either, but I'll answer the best I can.
1) Well the stars are an "initial evaluation of the players skills". So they are general and I'm assuming that they might be inaccurate a bit, because if not there would be no point to grade ratings.
2) Yes, very much so. The boxscore could be from a bad game they had, or a good one. Maybe against a weak opponent, or a hard one. So don't trust boxscores and I know that for sure.
3) Basically his skill levels compared to other draftees.
4) I don't know but Pappy's probably right.
5) I don't think so. People are always saying this year was a bad draft or a good draft. So I think there's no specific distribution, but I'm not 100% sure on that one.
6) I don't know about higher level leagues getting better or worse, sorry mate.
7) Well obviously they'll be of slight different quality, but in general they should be approximately the same.
I hope this has been helpful, cheers mate!