It is all so interesting Franck! A real puzzle for us to try to solve.

Your last photo is a much better image, as you said.

I really don't have a clue about which theory might be correct, but I think we need to keep one thing in mind -- According to Hugh Roberts, the Duchess of Teck's collet necklace can be shortened, 10 of the collets being detachable.

Given that, can we be sure that Queen Mary did not wear it herself to the 1937 coronation? I don't know. I am simply playing devils advocate to try to sort out the possibilities.