How downgrade macOS to previous versions safely?
There is no downgrade option available which let's you 'go back to a previous macOS version'. Most Mac Apps automatically convert your Photo, Mail or iTunes files to a newer database version. So you won't be able to import Mails out-of-the-box from Mail (Mojave) into Mail (El Capitan).


You will need to backup your data, first 
Erase your disk or volume using Disk Utility
Reinstall from macOS Recovery or USB drive.
Move back your data from backup


There's no need to reinstall your system due to USB adapter issues. It rather sounds like a incompatilbity issue. Your USB adapter is too old. Try using your USB adapter on another friends, collegues computer for further testing.

The only guaranteed step is to erase the system, reinstall a clean OS and only migrate / import data that was backed up to the lower version of the OS or older. You generally can load an older backup on a newer system, you generally cannot load a newer backup level on an older system.