I'd rather not see the late Queen's main tiaras worn by anyone other than the Queen Consort. I think it's appropriate to keep their status at monarch/consort level. I include the Girls, the Vlad, the Kokoshnik, the Belgian Sapphire, the Brazilian Aquamarine and the Burmese Ruby (even though I dislike it!)
The Princess of Wales has use of the Lover's Knot, the Lotus and could also be permanently loaned the Oriental Circlet, Queen Mary's Fringe and the Boucheron (the latter being grand enough for the coronation).
Princess Anne and Sophie Wessex have her own tiaras so don't need loans, although I've said before I'd like to see the creation of an Edinburgh tiara for Sophie when (if) that Dukedom goes to Edward.
Eugenie, Beatrice and Zara could wear their wedding tiaras and Louise could wear the Halo or ideally the Strathmore Rose.
If Meghan attends then the obvious choice is her wedding tiara.