I suggest you use a Private Browsing/Incognito window for all of this

Go to guerrillamail.com and have it generate an email. [Don't close this tab]

Then go here http://www.xbox.com/en-US/Live/Account/MSA and click on the account.live.com link

It should bring you to this page http://i.imgur.com/RnajjYT.png

Use this site to get a BS Chinese address http://topren.net/travel/necessity/hotels/hotel_beij.html (The Zip is the 6-digit #).

Fill in the credentials, use the guerillamail email, and make sure the country is set to China.

Click "I accept" and take care of any other things.

When done, and your XBL account has been created, visit the DL page http://marketplace.xbox.com/zh-CN/Product/Assassins-Creed-III/66acd000-77fe-1000-9115-d802555308ae?noSplash=1

If they generate a confirmation code, visit the guerillamail tab and find the email with the code in it.

Translate and "Purchase". Enter in any fake info it asks for with the help of the hotels list above.

Once the game has been "purchased", go to your xbox, sign out, then sign into your Chinese account.

Once that has been taken care of, press the middle button and go to Account Management. Then find Download History and find the Assassin's Creed III game and download it to your console. This game will work for any account on the console.