Camilla
What Camilla will wear depends ofcourse wether she will be crowned Queen or not. If she is crowned Queen, she'll likely wear George IV State Diadem on her way to the Abbey and be crowned in a Queen Consort Crown. I think they will use one of the existing Queen Consort crowns and modify it for the occassion.
If she is not crowned, i can see her wearing the State Diadem anyway or the Delhi Durbar Tiara.
Kate
I can see Catherine wearing the Cambridge Lovers Knot Tiara or maybe the Girls of Great Britain and Ireland Tiara for her first time.
Harrys wife if he is married
Possibly her Wedding tiara or maybe the Lotus Flower Tiara
Beatrice
Her Mothers Wedding tiara
Eugenie
Halo Cartier Tiara
Sophie
The Aquamarine 5 Stone Tiara or the Aquamarine Necklace Tiara
Louise (if she has turned 18)
Her mothers smaller Aquamarine Tiara
Anne
Double Meander Tiara or maybe the George VI Sapphire Tiara
Foreign Royal Guests
Mary
The Ruby Parure in it's grandest setting. Unless she is Queen, then she might wear the Pearl Poiree Parure.
Mette-Marit
Queen Maud's Pearl Tiara in the large setting. If she is Queen, she might wear the Emerald Parure instead.
Victoria
Almost impossible to guess. But I think it's pretty good chance that her mother will lend her either of the Leuchtenberg Sapphire Tiara or the Nine Prong Tiara for this extremly grand event. Unless ofcourse Victoria wants to use her signature piece the Baden Fringe Tiara
The Braganza Tiara is out of the question unless Victoria is Queen by then wich she won't be unless a tragedy occurs. I know Crown Princess Louise wore it to the coronation of King George VI in 1937 but she was the first lady of Sweden since 1930 and she was herself a junior member of the British Royal Family. So there was a huge difference that can't be compared to Victoria.
Laurentien
As monarches usually do not attend I think Prince Constantijn and Princess Laurentien will represent Netherlands. Maybe Catharina-Amalia if she has turned 18 but i think Constantijn and Laurentien is more likely. I guess she'll wear Queen Emmas Diamond tiara or maybe the Mellerio Ruby Tiara.
Belgium
No clue who will be in attendance. If anyone at all (judging from the history between these 2 royal houses)
haha. If The Duchess of Brabant has turned 18 she might attend. Or maybe King Philippe will send one of his siblings. Then I think Princess Astrid is more likely than Prince Laurent.
Luxembourg
Gulliaume and Stephanie are given. Hard to guess what she will be wearing. Maybe the Chaumet Emerald Tiara...
Lichtenstein
Probably The Hereditary Prince Alois and The Hereditary Princess Sofie.
Monaco
Prince Albert or Princess Caroline. I wouldn't mind if Albert stayed at home and sent Caroline to represent him. Then we will get Monacos largest jewels
hehe