Steam is required. You can play Steam in offline mode, but the games still launch the Steam client if you try to start their executables (well, some just displays error messages).
Correct.
I've run a game when Steam's servers are down for maintenance - i.e. Left 4 Dead 1 - from its EXE shortcut off my desktop and...yup, it boots Steam program up. Steam's offline mode for me has worked fine, when needed - knock on wood!
It wouldn't connect to Steam's servers, obviously - but the Steam program itself did boot up.
