Thank you Arthur for all those pics, and for presenting in sequence. If I understood you correctly, the tiara was not first worn for the Durbar. That fits my feelings and recollections.
When Queen Mary acquired the Cullinans she might well have spent considerable hours (?) inventing ways to wear them, and a grand head ornament might well have been in her designs. Perhaps the tiara was created for the Cullinans.
Edit: Note that for the State Opening of Parliament, Cullinan III is pendant on the Durbar necklace.
Edit 2: It is true that Cullinan III upside down on the top of the tiara is not very attractive. However, if it was replaced by Cullinan IV on the angle and then hung below as a drop within the design then that would be much better. It would fit that central space quite well.