A well written and really interesting post!
I never heard of the Republic of Minerva before. But I know of some other attempts to establish a independant new country - mostly they didn't succeed. Basically it takes other countriess to support the claim, and that is difficult to achieve.

Also, congratulation for aquiring this coin. I wonder what you payed for that. A issue of 10.000 is certainly not much, plus the peculiar background story it has.
The darker colour on the rim could be the normal silver tanning. Perhaps it was on the entire coin and somebody cleaned it. It is difficult to get it out of the grooves though.
Btw., karat and carat are 2 completely different things. Karat measures the purity of gold and is manly used in english speaking countries, but not really for coins.
Generally, they seem to have tried to be different with that coin. 24 grams is more or less 3/4 ozt of silcer, and here they used the common .999 label. But for the gold they used something entirely different. Weird...

But over all it was perhaps for the best that the Republic of Minerva failed. At the very least it would have messed up a coral reef, if it had succeeded. And much likely even worse things may have happened, as they did in other experiments of that kind.