Thank you for initiating this thread Arthur and for an interesting summary of the glitter still present at this occasion. I was saddened to hear that this might be one of the last times (if not the last time) we will see this many tiaras worn at the State Opening of Parliament. It was already hard for me to swallow the disappearance of the the Royal Princesses and Duchesses at this important date in the Royal calendar.
The lady you were referring to sitting next to the Duchess of Wellington is Countess Howe wearing the Howe Fringe tiara:
*from Tiara Mania's excellent site:
https://tiaramania.tumblr.com/post/756358122920198144/this-was-another-surprising-tiara-yesterday-the Another peeress I was able to identify is the Lady True. She is the wife of the now former Leader of the House of Lords. Sitting quite far away from the throne (almost all the way to the opposite side - on the third row between a lady dressed in black and a lady in pale blue in the first photo you posted) she can be seen wearing the lovely Diamond tiara she also wore the to the State Banquet for the Emperor of Japan last month.

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The times when the side benches of the House of Lords were packed with royal princesses and wives of peers donning their best jewellery are gone, alas. But there are still a few wives of peers wearing their jewellery, who are visible on the third and fourth side benches (on the right of these pictures).
I have counted approximately 10 to 12 tiaras, but unfortunately no close-ups. The only exception are these two pictures of the Duchess of Wellington, née Princess Antonia of Prussia. She is wearing an impressive suite of jewellery, comprising a floral diamond tiara, a large brooch pinned on her collar and a pair of large baroque pearl earrings:
Last year too, on 7th November 2023, the Duchess of Wellington was the only peer's wife who had been photographed in close-up. She had then worn another diamond parure from the Wellington collection (including the tiara that she had worn for her wedding to the current Duke of Wellington, then the Marquess of Douro, or for the bicentenary of the battle of Waterloo on 18th June 2015):
Both diamond parures are Wellington heirlooms. They are both worn on the picture below by Kathleen Emily Bulkeley Williams, wife of the 4th Duke of Wellington (the floral tiara worn by Princess Antonia in 2024 is used by Duchess Kathleen as [the main element of] her corsage brooch):
The lady seated immediately next to the Duchess of Wellington (with a black and white dress) seems to wear a very substantial (fringe?) tiara too. Does anyone know who this lady is?