Thank you Beth and Franck for sharing these images with us. And I think you are very wise Beth in putting the text on the photos so that Franck's contribution to this can be recognised - now and most of all in the future (when the images no doubt will start to made their way on this wonderous and tricky thing we call internet).
Personally, to my (poor) eyes the tiara doesn't look like a fringe tiara. It is sort of missing the graduation in size from the centre to the sides and there seem to be too much of an open space in between the vertical lines of diamonds. Also the very top line of diamonds (horizontal) seems to have sort of a larger element every so often, almost like there were larger diamonds set there.
So to my mind the most likely tiara worn at this event must be the double meander tiara.
It is indeed an important and exciting find and it will be interesting to see what others make out of it.
As for the necklace I do not have any ideas to be honest. They could be several diamond necklaces worn at different lengths, though my first inclination was actually the Greville 5-row Diamond necklace. But as you said, Franck, that would be unlikely since Mrs Greville was very much still alive at that point.
Any ideas on the brooch worn on the Sash by the way?