I have found a work-around; I did some unplugging and replugging and pulled out some drives that were unformatted and not being used and then created a backup folder next to the documents/mod folder and copied all *.mod files into it, then (offline) I started the game, the files in the mod folder vanished, closed the launcher, copied the files back, restarted the game (offline), the files vanished, stopped the launcher, recopied the files, restarted the game, files vanished. So; without stopping the launcher I copied the files back and then hit 'continue' and the game proper starts up my previous game with the mods working.
No, I don't find a difference in speed when playing offline or playing single player whilst connected, rather decades of slow downloading has left me tending to play without turning on the modem. I only turn it on if I want to actually get something, do mail, etc. So usually I play offline and unconnected. That, plus to use the modem, which is by the front window where the phone line is, I have to use an extension cord from the kitchen's one plug, cord is shortish so hangs across kitchen (and path to front door and bathroom) about half a metre off the floor. It's not an issue for me because every other game of mine plays offline without any issues.
And yes, if I want to youtube then I have to put up with buffering (until I get magical NBN) but hey, the copper wire outside my place was concreted directly into the ground in the days before anyone thought using conduit might be a good idea, so it has not been upgraded since it went in during the fifties or sixties.