I totally agree with your opinion about the brooch at the centre of HM's dress Arthur.
I have been searching on European sites for more photos of this State Visit to France - but unfortunately, I have yet to find any better photos.
I think - from what I have read in the British media - that the Return dinner and the visit to the Opera were on the same night and so the photo you posted earlier this year (or late last year) were of the same evening.