Reset wallpaper of 2nd monitor without having connection to 2nd monitor? How can I reset wallpaper on the 2nd monitor without having connection to that 2nd monitor?
You can reset them all by deleting this file

~/Library/Application Support/Dock/desktoppicture.db

Alternatively, you could try opening it in something like SQLite Free - but I'm not sure what the consequences of deleting a single line from the list of wallpapers would be.