Good for you. I can't. By the time I can run another qquest for them, I've already lost the karma I had with them.
How long do you play per day?
You need not much more than 2 hours of playtime a day. First action after login: Do the gunrunner package, then do whatever else you do and then last action after 2 hours do the gunrunner package again. It will be a while until you have the 1000 rep, but it will certainly not go down. Once you reached the 1000 you can do the quest once a day – maybe after an extensive crafting session – and you're good.
Yes, it's a chore, but you'll have the safest adv wb available. Safest does not mean 100% safe of course *cough*thieves*cough*

As for Mariposa:
My experience there so far is that 90% of the time there's no one else, soo i guess you had just bad luck getting killed.
Sierra:
I don't like how it has been hidden. If it wasn't on the wiki, who the f would have the idea to talk to npcs (hermit) in random encounters???