Modding the main campaign is known to be tough, mostly because you can never be sure of the loading order of mods when the game starts.
This means that, if you duplicate an existing resource from the campaign to your mod folder and alter it (say, for example, change the position of an existing NPC) it will only work half of the time, when your mod is loaded AFTER the campaign.
When your mod is loaded first, the main campaign data will overwrite the position defined by your mod and you'll see the NPC at its old position in game.
The only approach that works every time is to make sure that you only add content to the campaign and don't alter/remove anything. The trick is to remember that the added content can be a script that alters/removes another content of your choice. If you trigger the script when the game is fully loaded, you can be sure that it'll be executed AFTER all other mods (including the main campaign and your mod) and that it'll have the final word.
