One of the best companion stories in FNV is probably Boone's. His backstory of being a former NCR sniper and his quest related to his wife's death is really engaging. It adds depth to his character and the overall lore of the game.
Well, 'The Courier' is purely fictional. The plot and characters were imagined by the writers for entertainment purposes. There's no real-life basis for it.
I don't know. It might have elements of truth or it could be completely fictional. You'd have to look into the background and author's statements to know for sure.
Not really. 'Courier' is mostly a fictional creation with elements that might draw inspiration from various sources but not based directly on a specific true event or person.