Such an interesting theory OmerH! The flower looks remarkably similar to the work of Belperron. White coral could have been used, but I’d rather think it is made of rock crystal or chalcedony, like the examples shown in your post.

I’m quite surprised that this brooch hasn’t been seen recently, especially given the huge interest in the work of Susanne Belperron.

I don’t have Olivier Baroin’s reference book on the subject, but I guess a Royal connection (other than the Windsor’s) would be mentioned.