If you’re unsuspecting and the investors get enough return, then it’s really not to hard as no one is going to be watching you or have any reason or ability to audit your finances.

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.

Well you use your real share to generate a complex series of investments to make it very hard to judge your wealth. In it you begin trading gold legitimately, not gold markets but buying and selling real gold. The stuff off the boat gets mixed in over the years. Buy some shipwreck gold! Collect gold coins personally! Buy and sell collectables

Melt some down, have it remade into other things, and it gains a bit of weight here and there.

Melt it down and sell at spot price would be my guess

I mean it is pretty easy to sell gold, especially if you sell it below spot price or melt it. If someone offered me gold at a 10% discount, I would immediately buy it and go sell it at whatever gold dealer for an easy profit. Im sure most gold dealers would do the same thing even if it was obviously stolen spanish gold or something.

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.