I was very young at the time so I really have no idea who Alexandre de Paris is/was!
About the mirrored necklace, I have been thinking for some time now if D.Amelia had not it dismantled in order to create the diamond choker/tiara. After all we know that both D.Luis diadem and the coker/tiara arrived to the present Duke of Braganza in the same box (even though not the original one).
However according to the book about the portuguese royal jewels the mirrored necklace had about 500 diamonds and the diamond collier de chien does not appear to have so many diamonds. Besides, Queen Amelia was not known for playing with her jewels like Queen Mary of UK, that was always changing her jewels.
The sapphire necklace disappearence is tragic: I still believe that the portuguese royal collection should have more sapphires.