Your unencrypted wallet.dat sounds pretty messed up if not even one 'key' related entry was found.

Take a look at this https://github.com/bitcoin/bitcoin/blob/48b5b84ee511d5ccd0d47bb0018c1b3c9ddebeff/src/wallet/walletdb.cpp
There should be plenty of entries starting or ending with 'key'.

Guess you have to resort to: https://walletrecoveryservices.com/  afterall.