If I am not mistaken, Queen Sofia attended only one tiara event since the abdication and, on that occasion, she wore her Ruby Necklace tiara.
I suspect the Prussian and the Shell tiaras will be given to Princess Leonor when she makes her official tiara debut. I don't see Leonor getting a new gift tiara until she gets married. The Greek rubies will likely be left to one of Queen Sofia's daughters, probably Infanta Cristina since Infanta Elena apparently kept the Marichalar tiara following her divorce.