Well, good points Brian and thanks for displaying the photos so that the tiara in the Moika cache can be seen more clearly.
You are absolutely right about the photos of Irina; her dress and coat are very much after 1920 and therefore could not have been taken say, during their honeymoon, which is when they are presumed to have collected Irina's reset jewels.
Having said that, it is documented that the Chaumet tiara was comprised entirely of diamonds and contained no central dark stone; I think that you have to agree that the jewel in both photos is virtually identical in style. Perhaps a case may be made for Chaumet having made a copy, either at the same time as the original or perhaps a little later.
This seems to have been common practise for jewellers; after all Cartier took the opportunity to make copies of Miechen's diamond and pearl circle tiara (now owned by QE 2) when she left it with them to be cleaned.
According to Diana Scarisbrick's "Timeless Tiaras" Chaumet have a huge archive that contains "37,000 glass negatives, 80,000 drawings and as many photographs, and replicas in nickel silver of the 1,500 tiaras made over the last two hundred years." Thus a copy of the mount of Irina's tiara would have been in the archive and it would have been easy enough for Chaumet to make another identical tiara if required. Equally, a copy could have been made using paste substitutes; there are therefore arguments to support the existence of two tiaras that fit the description.
Interestingly, this particular jewel was not unique; Geoffrey Munn's book "Tiaras Past and Present" illustrates a very similar platinum sunburst tiara made by Cartier in 1927 for Countess Tysckiewicz, comprising alternating bands of brilliant and rose cut diamonds, with a central element en tremblant set with a star sapphire. Apparently several other versions of this jewel were made by other Parisian jewellers at this time.
Interestingly, Chaumet also remade for Irina a ruby and diamond garland-style tiara, a bandeau centred on a huge emerald and an aigrette set with the Polar Star diamond, all from existing Yusupov parures and at the same time as the sunburst tiara. I wonder if these can be found in the photo of the Moika cache?