Diablo – Do I need to own Diablo III to download and install the game

You can pre-download Diablo 3 once you've purchased it online. As @dlanod mentioned, you can generally pre-download Blizzard games.

However, to more directly address the question you have proposed you need to remember/be-aware that Blizzards downloader is a Peer to Peer downloading scheme. They use small feeders to populate different sections of the game to different people and then all of those people are offered up as hosts to everyone else to get those parts.

So the situation is not really a few hundred thousand people all hitting one or two servers for the game, but a few hundred thous and people all getting a small piece pretty quickly from the server like normal web browsing or MMO traffic or whatever, and then sharing that with everyone else as long as they are downloading.

There should not really be any release day concerns with access the server to download the client to your system.

Yes. Diablo 3 does not allow offline play. If you want to play with someone else, they must each make it past the log-in screen... which means you will both require a separate copy and associated Diablo 3 licence.

Also of note is that you may only have one Diablo 3 per Battle.net account, so your wife will need to set up an additional Battle.net account to be able to play with you.

If you purchase a physical copy, however, you will be able to share the installation disk, but Diablo 3 does not require a CD to run.