Well, in any case, if OP has managed to get as far as "decrypting" the wallet backup file using openssl... then they can very easily load their wallet/keys into a BIP39 compatible wallet
that allows you to specify the derivation path (such as Electrum). If you simply open the "decrypted" file in a text editor, you should see "org.bitcoin.production" (as you say you are recovering from a PROTOBUF wallet)... and then shortly after that, you should see 12 plain english words... this is your 12 word seed.
Here is an example of a decrypted wallet backup:
If you load that seed into Electrum... ("New/Restore -> Standard Wallet -> I already have a Seed -> Enter seed, click options -> select the BIP39 seed option) and then on the derivation path screen select "legacy" and then set the derivation path to:
m/0' [1] then it will load up all your old legacy addresses.
[1] NOTE: this derivation path is slightly different to the one used by ian coleman's tool below because of the way Electrum works
Alternatively, you can put the seed into iancoleman's BIP39 mnemonic code convertor... click the "BIP32" tab, and then select the "MultiBit HD" option from the "Client" dropdown box (This will automatically set the derivation path to
m/0'/0 for you). Then it will show all your legacy addresses/private keys at the bottom:
All your keys displayed below: