Steam – If I buy the same game I already have downloaded on the computer from a different steam account, will I have to re-download it

Playing Civ V in offline mode should allow you to play other steam games simultaneously. I feel compelled to point out, however, that steam only allows one copy of itself to run at any given time, so short of some technical tinkering, you're still limited to one game / steam / computer.

I don't think that there is a physical copy of civ V. Everything I've read indicates that its going to be exclusively digitally distributed.

As far as I know, Steamworks is going to function as the game's DRM which means that even if there's a physical install disc, you'll need steam to play it, even in offline mode.

Because of the way that steam does things it's not able to have multiple auto sign in configurations per installation. You'll have to resort to trickery:

• Disable account credential saving in the settings dialog.
• Create a shortcut for Steam on each windows account, edit the properties and give it these launch options -login %u %p. This logs into Steam with the specified Username and Password combination. Replace %u with the username, and %p with the password you want to login with (Steam must be off for this to work).

Source.

If you still want Steam to automatically startup, while logging in to the appropriate account, disable automatic startup within Steam and place the shortcuts in the Start menu → Programs → Startup folders of each windows account.