Provided your friend has not redeemed the gift, you can resend it to any email address you so desire. In fact, Steam recommends gifting a game to yourself if you would like to give it away on a later date, as all the other user requires is the unique key. However, once a game has been redeemed, it is permanent.
Source: https://support.steampowered.com/kb_article.php?p_faqid=549
As for the region, they will receive the version of the game from which it was purchased. In the case of L4D2, the recipient will receive the non-censored version if they are Australian but the purchaser lives in the US. There are no restrictions in sending gifts across regions.
Source: Experience gifting 32 games on Steam, including two L4D2 copies for Australians.