How to restore a backup made when using the big 20 Gigabyte Core Wallet to current MultiBit wallet?
First of all it is extremely dangerous to back-up your wallet to Dropbox or any other cloud based file service. You might find that if you do recover your wallet, it may have no more funds in it, but I'm hoping that you encrypted your wallet before backing it up to Dropbox and you can remember the passphrase.

Secondly, running vanilla bitcoind is not a mining version of bitcoin, by default, so I am assuming that you know what you are doing, although it might have cost you a considerable amount to run that on amazon. 

Assuming that you haven't been unlucky or naive, you cannot directly recover your wallet to Multibit, the wallet file formats are not compatible but in any case this is not what you should be doing. The simplest way is to make a payment from your running bitcoind wallet to a new address in Multibit.

It actually surprises me just how many people don't understand this.