You would put the ownership/authentication paperwork for the art in a third party escrow. Then you would have a set time to do the extensive research of verification. This would be carried out by a licensed appraiser, an advisor on retainer or by an auction house. Check with your lawyer to see if this would then still have to be included in the estate.
Fair market value assessment is based on an availability of market, so the assets could technically be listed as worthless until a market is found.
This would especially pertain to the Man Ray. An appraisal by a gallery in the 1990's cited fair market value at $20k, but a letter from the MoMA "prevents me from giving you any estimate of the picture's value". (i.e., "no one is gonna buy it")
The chess set is also NaN because you guys can't clear it up in the will who it belongs to. (i.e., "no one is going to sell it")
The other pieces may be of nominal value. The chess set may be of black market value but this would be a much lower estimate than a Christie's hammer price, so you would want to get a minimum range from them.