Here's the thing with all of this, and it's something I keep going back to. "If" this story took place during the period described only to resurface some sixty years later, regardless of the actual source, then some means of accurate record had to be kept someplace. This is issue one.
Issue two revolves around the number of people involved who clearly would have had a lot riding on the venture. So it is unthinkable that at least one or more of them would not have taken steps to protect their interest, this again creating some type of written/documented record/account.
Families.....certainly there would be some reference of either the adventure or the hidden wealth in family records and/or correspondences. This even being more assured had it been a type of business related venture.
Yet in all this time not one single reference?
So if it happened then there should be some measure of reference to it, somewhere. So, either we're looking at the wrong people or we're still looking in the wrong places?