ah, apologies that I haven't responded; we noticed this post but I was busy patching some other stuff ^^
for anyone else having the same issue, I'll elaborate: once upon a time, in order to play missions with mods, players had to copy the mod files into the folders and stuff themselves. it was a huge pain in the butt since you had to back up everything, and then replace later with the original files when you wanted to play different missions
Feda's upgrades to the Mission Launcher do this on the back-end, automatically copying mod files under CustomCampaignData where they're needed, when they're needed, and then replacing them with backup files when you're finished with the mission. this is how the mod files from one campaign get into your main data folder in the first place
as for why they remain there and why the Mission Launcher doesn't replace them with the backups, that probably has something to do with the read-write permissions of the install directory. I don't really know how it works, but what I do know is that trying different install directories or adjusting the permissions for the directory you install to can remedy this issue
I hope that helps anybody encountering the same issue