I once read that the Sapphire tiara came from Princess Louise of Orléans not sure how true that was.
Yes, that information is confirmed by this site .
Infanta Pilar's family seems to wear this tiara regularly though. Two of her daughters-in-law wore it on their respective wedding days and her sister also wore it in the past. I don't see it going back to the main line unless Pilar's children need to sell it and Felipe offers to buy it.