So if I understand correctly, you are saying you tried to send  .54497BTC to an address, but it was actually sent to a different address? was this the entire balance of your wallet?

I don't know if you can import private keys to coinbase, but I wouldn't recommend using coinbase at all. If you use coinbase they have full control over your bitcoins, they are essentially a bank and can freeze/close your account and they have done so to people who have used some services such as unlicensed gambling websites. Multibit is actually an OK client, though I would recommend electrum much more.