Ursula is to be congratulated on finding the images of the Queen of Hanover and Thyra, Duchess of Cumberland and noting the similarity of their necklace to the one exhibited in 1862. But is the Hanover necklace the one exhibited in 1862?
When did Marie, Queen of Hanover acquire her necklace? That might help solve the mystery. Ursula does not have a date for her photo of Queen Marie wearing the jewel. Of course a date for the photo does not prove when the Queen acquired the necklace, but it would help in putting together a time line.
When Princess Thyra of Denmark married in 1879 among her gifts were a necklace and bracelet from the Queen of Hanover described as being of rubies, diamonds and pearls. Perhaps the necklace was the one under discussion.
The Cromartie necklace proves that the design was not unique and that, at least two, if not three, such necklaces existed.
The Christie's site gives a date of 1849 for the creation of the Cromartie necklace/tiara. I wonder if they had access to papers proving that, or if that,like the legend that Marie Antoinette gave the pearls to the Countess of Sutherland is based on family belief? If the date of 1849 for the creation of the Cromartie necklace is correct, then the 1862 necklace is a copy of an existing design.