All you need to play the FAF offline is to create a shortcut to
and run it. If the game crashes, you may have to run
C:\\ProgramData\\FAForever\\bin\\ForgedAlliance.exe as administrator (this comes with a security risk, especially if you obtained FAForever from an untrusted source). If you run the game offline you will not have access to some features such as autodownload of missing maps/mods from the vault or automated replay saving.
If you want to play a coop mission in single player you need to "host" it first, so that it downloads the relevant files. Once you get to the in-game lobby you can close the game. You need to do this once for each mission you want to play. Every time you want to play singleplayer: Start the game with the /init init_coop.lua command line switch or shortcut. Begin the mission from the skirmish screen. Resume saved progress from the campaign screen.