Steam – sync Steam ESO files with the Bethesda client so I don’t have to download them again

I copy the files like you suggested all the time, and it is fine to do. Any files that do not match are usually re-downloaded from Steam, hence why it will still update a bit after a fresh copy. If you do not want all to redownload just the sounds and maps (which take up the most of the space), you can simply take the .gcf files for each game from Program Files/Steam/SteamApps.

Steam also has a "Backup" feature you can use. The files it creates can be moved from PC to PC and restored on any one.

Yes, you can do this. After playing the Steam-installed version of Frozen Synapse (so that you have a registered account and email address), you can go to Mode7's website and use the form there to request a link to download and install a copy of Frozen Synapse independently of Steam.

Note also that the Steam-installed copy can be run without going through Steam. Just browse into the install directory and run the exe directly. (Don't use the desktop icon created by Steam, as that runs it via a steam:// URL instead of starting the exe directly.)

I have personally used both of these methods, having initially installed FS on my dual-boot laptop via Steam, set up an icon to run it in Windows 7 without starting Steam, and also run it under Ubuntu without Steam even being installed. I then requested a download link from Mode7, used it to download the OS X version, and installed that on my MacBook Pro to run without Steam.