I should have looked at dates more closely! The photo I posted of Queen Ena wearing the ruby parure was dated 11 April 1931. Yet, the King left Spain on 14 April 1931, so I hardly think that Queen Ena was sitting for photographic portraits just a few days earlier, as obviously both would have been aware of the political situation, what the consequences of the elections would be, and how the monarchy would be positioned.
After reflection, I am still inclined to accept what Vincent Meylan said in his Instagram post and in his book on Christie's. He has an impeccable source. My experience with working with oral history is that, usually those involved have fairly accurate information/memories. It is just that, sometimes, a date range may not be accurate because a person may not have been personally involved.
In this particular case, I don't think that the photo of Queen Ena wearing the parure would have been taken before 1917, when her brother married. The style of dress is wrong. Or am I wrong about that? My knowledge of the history of dress is very, very limited.
There is nothing in the list of wedding gifts given to Lady Irene Denison for her wedding to the Marquess of Carisbrooke about rubies -- or even a tiara. I don't find that unusual as they married in 1917 during WWI, when newspapers were not reporting in detail about wedding gifts etc.
Perhaps this will have to remain a mystery -- unless some other evidence becomes publicly available.