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.
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.