To Clean Or Not Clean ?

Do YOU believe there is residue? It sounds like others do not and I don't blame you for being very careful with an expensive coin.

Have you considered sending it into NGC or ANACS before attempting a cleaning?

I regularly run into Morgans which show no PVC activity to the naked eye, yet under the high magnification of my imaging equipment it pops out in small spots. It's quite possible that PCGS saw the same, and you can assume that they aren't wrong. This is a problem which must be corrected. It's a progressive process which can only be fixed by removing the PVC; otherwise the damage will eventually become permanent and disfiguring.

1) Is the coin silver? Acetone is chemically incapable of reacting with silver, even alloyed silver. It is completely benign in that application. You - or your designated "conserver" - can use it with confidence on a silver coin.

2) Is there any non-mint color on the coin which can't be explicitly identified as natural toning? Acetone has absolutely no effect on natural toning as that is essentially still silver. However, if there's anything on the coin which is postmint and not silver, acetone may remove that and leave an unnatural patch of color because the surface underneath has not aged like the rest of the coin.

If the answers to the above two questions are favorable, I would have no qualms about treating the coin myself, even cognizant of its' value. Not being expert in its' use, you may have a different level of tolerance. Either way, act on what PCGS is saying. It's a safe, simple process, to the extent that many collectors (myself included) bathe every single eligible coin they acquire in acetone just to be safe.

I am adamant; although no chemist, I am knowledgeable in this specific circumstance. I have to reiterate, though - the coin must have nothing external and post-Mint on it because of my previous caution about removing something which would expose surfaces which haven't aged at the same rate as the rest of the coin. I'm going to guess, given your evaluation of the coin, that nothing such exists.

If that be true, if the coin is either original luster or only natural toning, neither you nor PCGS will see any difference whatsoever in the appearance of the coin after the acetone. That is simple fact.

In your shoes, as long as the cost is within your comfort zone, I'd submit it (in the current slab, don't crack it) to PCGS' conservation service. I'm surprised that they did not make this recommendation to you when they determined PVC was involved.

Are you willing to specifically identify the coin? "No" is a perfectly understandable answer, reticence being always a virtue when discussing high-end coins, but knowing that might enable me to more specifically tailor my answer as differing coins tend to age differently. I - or someone else better-situated than I with that particular issue - might be able to more clearly state what should be considered "normal" with the current appearance.

No, you're right. I retain my title as CCF's most prominent proponent of incomplete reading comprehension.

If the coin really is that valuable, I'd send it to NCS.