Couldn’t there be some statue of limitation about this? It was in their possession...and they can’t prove that the present owners had stolen them...
The narrative in the story is one of theft, plain and simple. The 'lucky' finders of the coins were idiots to take ALL of them to the Mint.
Proper strategy: take (don't ship) 9 of them to, say, Nevis (which does not recognise civil orders of recovery from ANY other nation). Then, take the remaining coin to the Mint. Have the Mint issue an opinion of authenticity, and then have the remaining coins AND the opinion (or certification, perhaps) of authenticity auctioned by a reputable international auction house.
Oh, and don't forget to expatriate (lawfully) prior to the auction. This way, the coin stolen by the Mint effectively becomes just 'taxes paid'.