I haven't seen the code you had to port & integrate. But from your description it was little-endian single-threaded C code not using exceptions. Here you have many more additional dimensions of complexity.

Satoshi bitcoin client is really a trailblazer in the annals of bad software design. Can you name any other software that uses public-key cryptography and stores both public and private keys in a single object? I presume most people got acquainted with asymmetric cryptography with PGP. Even then, about twenty years ago, there were two separate files: PUBRING.PGP and SECRING.PGP.

Now you have people saying single WALLET.DAT "isn't all that bad" design. I just checked, you think so too. Who erased your memory?