It looks like multibit classic wallets can get (easily?) corrupted. You should ask the creator of Multibit to help you.
I managed to find out that between 2 different versions of wallets the data between offsets 0x3a and 0xae is unchanged, so your private key may be there. Of course, I have no idea how to decrypt it.

So I'd say:
https://multibit.org/help/v0.5/help_contents.html
https://github.com/keepkey/multibit/issues
support@multibit.org

Good luck!

Edit: this may be of better help https://multibit.org/blog/2015/02/02/wallet-forensics.html