How can I make MultiBit go one year back and then replay the blockchain?
When you import private keys, you specify a replay date for each private key. This should be before the earliest transaction involving that address. In your case, it appears that this is not the case. (Or maybe you just need to reset again - their site says it might take up to 3 resets for the balance to be corrected.)

You should reimport your private key with the correct date specified to have the balance appear. Note that having your private key(s) in plaintext on your hard drive makes them an easier target for attackers. To be most secure, you should consider those keys compromised afterwards: securely delete the file when you're done with it, and transfer the coins to a new address and avoid using those old addresses again.

Another way of transferring the balance is to send it from your out-of-date Bitcoin-QT wallet to an address generated by MultiBit. I'm not sure if that's complicated, the wiki makes it appear simple.