The status of Diana’s jewels is interesting and I doubt that Diana’s jewels formed part of her estate on her death.
Under English law her engagement ring was her property. The wording in the article is also important, noting the tiara, brooches etc were “gifted” to Diana. Under English law therefore she owned them. Of course, it is quite normal as part of any divorce negotiation to deal with chattels such as jewellery and for such items to be passed from one party to another.
She could have gifted them back to the Queen in 1996 to secure the inheritance/CGT tax exemption the monarch has (the article implies this to my reading) or she could have given an undertaking within her divorce order to bequeath them to the monarch (whoever that was at the time of her death) in her will, thereby also securing the tax exemption.
No one would of course anticipate that Diana would die 12 months later.