How did he get in this mess? I reckon a complete moron could have stolen 10% of the gold without getting caught. Just count and log less than you found. Drop it overboard with a GPS pin on your way back to port. Get it next year.
To your other point it wouldn’t be too hard to sell 10% of the gold to dealers with the proceeds /paperwork unaccounted for / off the books.
Melt some down, have it remade into other things, and it gains a bit of weight here and there.
If you wanted some sort of legal paper trail to fall back on you can spend a bit of money on a few machines and rights to a gold claim in Alaska, dig some random holes for a few months, then claim you struck it rich.
I’d say it’s almost expected that someone in that position would skim off the top in this way. The problem is he stole it all and that’s obviously going to raise questions.