Noted with thanks Kongetro.
Thank you for raising such an interesting subject! But first just a small correction: the lady wearing the Mellerio Shell tiara is in fact Infanta Isabel, the daughter of Queen Isabel II. The tiara was bought by Queen Isabel II as a wedding gift to her daughter Isabel in 1868. So it is not very likely that the Queen ever wore that particular tiara.
Also, the first photo when talking of the many pearl necklaces and brooches, is again in fact Infanta Isabel, not Queen Isabel II. Having said that, the Infanta did leave some jewels to her nephew King Alfonso XIII. And she might of course have received them from her mother at some pont. So the pearls might have been part of the jewels King Alfonso XIII received and as such it might be possible that they have still been retained by the Spanish Royal Family.
The collet necklaces now worn by the Spanish Queen started off as a "small" collet necklace offered by King Alfonso XIII to his bride (Victoria Eugenia). In this photo one can see how short it was (it is the one worn at the top, almost like a choker):
Over the years the necklace grew in lenght as King Alfonso added to the number of diamonds by offering the Queen two diamonds each time he wanted to give her a gift, for birthdays, anniversaries etc. At some point Queen Victoria Eugenia split what has then become a very long necklace into two separate necklaces. Again, it is possible that the diamonds came from any inheritance that Alfonso might have received but I don't think that this has been possible to establish 100%. It is known that Queen Isabel II had to sell quite a few pieces during her time in exile.