Comparison photos.
1. Mrs Ronald Greville v Lady Greville
2. Princess Eugenie v Lady Greville
I can see several more differences.
First I would like to point out again the different sizes of the central element. In the Boucheron tiara worn by Princess Eugenie it is obvious that the central element goes from the top of the jewel to the bottom. This is not so with the tiara worn by Lady Greville.
Secondly, the Boucheron tiara worn by Princess Eugenie is more graduated from the front to the back than the one worn by Lady Greville.
Thirdly, to my eye it looks as if the emerald side elements in the Bouchern tiara worn by Princess Eugenie are smaller than those in the one worn by Lady Greville.
I would stress again that the design of diamond section of both tiaras looks completely different.