Thank you very much for this interesting post, Jorge!
I do like the tiara as worn in the official portrait very much. (Not so much in its more closed form as worn at D.Isabel's wedding - and as lovely as the Duchess always looked, the proportions of her wedding dress, hairstyle, awkwardly positioned tiara were, IMHO, all wrong for her.)
Thank you also for clarifying the ownership of the tiara. I always assumed that it was part of the Foundation, simply because the tiara was never worn at any occasion the Duchess attended since her wedding. The question remains: why? The piece has just the right dimension - substantial enough to make a statement, but not overwhelming a "petite" woman like D.Isabel.