Steam has some backup feature. I do backups of my games. Compressed to slow HDD dedicated to backups. Faster to install than downloading again from network. Don't know though how much will that help

Can one download a backup first, then install (using usb drive) into an "offline" machine first (without steam/online?)

Yeah, if the game is set up to not use steam as drm. You can check by going to the installed files and running the exe directly - in most cases it'll still launch even if you're signed out.