When it comes to the tiara, I'm in team Luxembourg.
Imo, it looks more balanced with the central diamond prong, and it's usually the best look for a tiara to have the highest spot right in the middle. On the other hand, I'm delighted to see yet another version of this gorgeous and historic jewel! And I wonder, if the side prongs are removable as well, as that might make another pleasing, more bandeau-like version.
The earrings were a bit too much for my taste. While being gorgeous in themselves, they look so chunky that they are a bit lacking in elegance. Queen Maxima wore some smaller (yet still big) diamond earrings at the British SV with the large setting of the tiara, and I prefer that look.
As for the Stuart diamond in a brooch setting: I believe that a custom made one shoulder dress, like the one she wore, could easily be strengthened at the shoulder in order to support such a heavy brooch. But this particular gown with it's rich embroidery was probably best left brooch-less.
PS. I agree that the hairstyle complemented the tiara perfectly.