Yes. Except I'm not even sure you can activate a Steamworks game's disc multiple times - I think it's one key per disc and you might have to buy a second copy off Steam/at retail and download it yourself.
Yes, that's how it works. In exceptional circumstances, you may be able to get Steam support to revoke a key and apply it to your account, e.g. if you purchased a brand new game, the key had already been activated, and the retailer won't take it back.