I tried using 2 versions of Bitcoin Core (0.21.1 and 0.8.6) to read the wallet.dat and was given the warning "wallet.dat corrupt, salvage failed".
If you see error message when you open Bitcoin Core normally, you can external Bitcoin Core tools to salvage your wallet properly. Check this thread,
Salvagewallet in latest Bitcoin core.
My next step is to recover the HDD using GetDataBack instead of Recuva, and then reattempt what I have done so far.
I strongly suggest you to make RAW copy of your HDD using tools such as
http://www.hddguru.com/software/HDD-Raw-Copy-Tool/. After doing that, you can use the software to scan the RAW copy instead.
If you're searching text with a hex editor then you need to type key!. The exclamation mark is important, but is not used for the master key (mkey).