(3) is much easier to implement for Ubuntu due to PPAs. It's also the most convenient for the users, although trusting a wide range of PPAs is somewhat of a security issue.
Most likely, Valve will provide a steam_installer.deb, which depends on all platform libraries Steam needs, and which only contains a script that creates the necessary users and rights, reserves some directory for steam, fetches the actual Steam off a Valve site, authenticates it, and runs it.
I guess this is for performance reasons, but I'm sure my PC can run Braid and do some disk IO at the same time.
I also can't find a way to have different games installed on different partitions/drives. This would be very useful if you have an SSD but can't fit all of your games onto it.
You can do this by moving the directories in question into the other disk, and then creating a junction (symbolic link) from the original location to the new one. On windows 7: mklink /j "path/to/steam/steamapps/common/gamefolder" "path/to/the/other/disk/gamefolder"
Shame this isn't made a bit easier.
You can do it manually. Activate the game to play, alt-tab to Steam. Library, view the downloads. The download will be suspended, pause it and then you can unpause it and it'll resume downloading. Alt-tab back to the game.
Lots of work, but it works (or at least I think it did when I last tried it).