If I were you I would take your backup of your private keys (you certainly have one, right?) and simply import that into a fresh wallet program (multibit or some other program) and that should be fine. You should be able to verify that your addresses still have your bitcoins by looking at a block explorer like blockchain.info. After that, it's just a matter of importing those keys into some fresh, unborked system and retrying the transaction.
How you actually got borked is not really a question I can answer.
I created a wallet at blockchain.info since they seemed to be able to import MultiBit's private keys, but now apparently they don't anymore (they told me so). Any recommendation about a OSX or web client that could import it?
As far as I know, any wallet program with any import functionality should be able to import any private keys (right?), it's really just a matter of formatting. I would definitely stay away from web-wallets if I were you. Thousands of bitcoins have dissappeared when these websites get hacked/go under/etc.
I've only used bitcoind, multibit, and schildbach's android wallet and I've never had any issue importing/exporting keys between these programs.
Since you're having trouble with converting the format of keys, why not just re-import your multibit keys into a fresh wallet in multibit?