Ok, well here is the word from my local post office.
The counter lady immediately complimented my honesty, she said most people would have just kept the item. The USPS clerks can see the tracking number or access a more in-depth search at a computer in the back area. However, she said that the Missing Mail Request would simply show her that the package had arrived, and maybe some details of how it was found at the problem facility. However, what it would not show them is the insurance status and whether a claim had been paid out.
She also said that there's really no standard operating procedure for a claim that has been paid, and then the package shows up much later. It's a gray area that just happens very infrequently. I personally imagine that there's a bulletin somewhere that gives guidelines, but nobody has them memorized because it rarely comes up. I didn't find anything on a Google search, though.
If the claim has not been paid (acknowledging that there is no way to see the claim status as the buyer), then the package is rightfully the property of the seller and should be returned.
IMPORTANT: As long as you do not open the package, you can take it to the USPS desk and refuse delivery. That will return it to the sender using the same tracking code, and you can track the progress back and legally verify the return. Once you open it, you have to ship it yourself with a new code, and the chain of custody is technically broken.
My next step is to contact the seller. The clerk acknowledged that if the seller has been paid the claim, that he could easily double dip by having me return the coin, or having me pay for it again.