If you're playing A21 and want to stay on A21, then in your library right click 7dtd and on settings/betas set beta participation to Alpha 21.2. Then you'll stay on alpha 21. No messing around with offline mode.
You can copy whatever version you want, rename the folder and set a short cut to the launcher so you can then update to V1.0 if you want and have both versions playable at the same time.