I am continuing to look into this in my free time (I don't have a lot of it). I believe the save file is utilizing AES encryption which requires 2 keys to decrypt. One should be hard coded into the game and the other should be within the save file (I believe the next 16 bytes after the header could contain that key). I need to disassemble the game and see if I can find anything. I didn't think they'd go this far to protect a save file when plenty of other first party releases aren't obfuscated whatsoever.
To be honest I'm more interested in figuring out how to modify the pro rank for a sport. I don't know how I keep matching against absolutely amazing tennis players who can hit a perfect serve every time, but I keep getting screwed and can't get past C tier.