In regards to the failing, due to IP check - If you've already got it on your download history, has anyone tried doing a account migration?
https://live.xbox.com/AccountMigration
Came up on a GameFAQs topic about it. Assassin's Creed 3 is linked to the profile, that's set. The profile is based in China, which obviously isn't USA/UK/wherever you are.
If you do a migration to your home country, the IP check will pass.
My internet is horrendous (I download at 400 kb/s), so I won't be able to see if my download passes or not (currently at 35%), but this may be worth looking in to. If it is 'corrupting' at 80%, you MAY just need to download the remaining 20%, or possibly the whole thing.
I'll check this topic tomorrow morning when I wake up, to see if anyone's tried the migration!