For your troubleshooting example, I use
grepWin with a "Search in" string of "D:\World of Warcraft\Interface\AddOns|D:\World of Warcraft\BlizzardInterfaceCode". A google search for "BlizzardInterfaceCode" should find a pointer to the instructions on how to extract that data from your WoW client.
A search for "You are not in a raid group" finds ERR_NOT_IN_RAID in the Blizzard file GlobalStrings.lua and a search for that should find the addon which is spamming you (its not in any of my addons).
I believe
Garrison Mission Manager has the ability to mass add and remove building followers and I use
Broker Garrison to quickly see who has what buildings.