Does anyone know if there is a mod so that it makes Quest NPCs unkillable? If so a link would be appreciated. I've searched Google and didn't come up with anything so i'm asking here.
Like Ryu said, I don't know of any mod like that however, when you accidentally kill an essential NPC the game gives you a warning that you screwed up so you can go back to a previous save (you should save often and on different slots)
I don't know of one but it could easily be made. I'm not saying there isn't one but I don't know of one. You could just make them all have like a zillion health in the CS. It'd take awhile. I'll search around for you real quick...none that I can see. I'd go through and make a mod like this myself but I don't know if it causes issues...
This will cause issues with the post-endgame quests of at least one faction as far as I know.