Some answers to your questions:
1. I don't know exactly, but killing Slavers brings your reputation up with some factions, killing Enclave bring it up with BoS, and killing BoS brings it up with Enclave, I think.
2. 0 points, not positive 200. The other way is to join a certain NPC faction. Having good reputation really has no good affect though, unless you are Idolized, which I believe is impossible except for Vault City faction.
4. I don't think it affects it. It only affects time you can barter at one time.
5. Killing raiders in the northern base will bring your reputation up with a lot of factions (not above 0), last time I checked. But only in the base, not in random encounters.
6. No, there isn't.
NPC factions are going to be re-done in the coming update, it's been said, and the mechanics will probably get changed around a lot anyway.