1) You can only access one game per account at any one time unless you set the computer to offline mode.
This means that if you have 2 people in the house, and one person wants to play TF2 (online) while another wants to play CS:S (also online), you can only do so if they are on individual accounts, e.g. TF2 on "ZeboTF2" account and CS:S on "ZeboCS:S" account.
If they are both on the same Steam account (say "Zebo"), you cannot play both at the same time, even on different computers in the same house/on the same internet connection.
If you want to play, say, Bioshock on one PC and Half Life 2 on another (two single player offline games), and both games are tied to the same Steam account, you can log into Steam on one computer, then go into Offline mode, and then log in on the other computer and be able to play either game on either computer at the same time. You could even play Bioshock on both computers at the same time if they are in Offline mode.
You can install all games onto all computers though (non-Steam DRM excepted) and access any and all games on that computer 'just' by logging into the relevant Steam account. Any other computer also logged in to said Steam account will be logged out.
2) No, never. All you get is a copy of the game to download which is attached to the Steam account it's bought with or for (since you can gift games).
3) They have in the past said they will try and make sure you don't lose access, but since it's not happened yet, there's no way to know. Just consider the games long term rentals.
2b) Hard copy games which you purchase that require Steam (e.g. any Valve games, CoD:MW2 etc) are useless and unusable without Steam, so there is no real "hard copy" anyway.