Thank you Helen for your work. I had not yet turned to the same task.
I felt it was a relatively short period in which the Cullinan, Cambridge, and durbar needs all came about.
Yes, the King had a new crown made - with 8 half arches - because he could not take the Imperial State Crown out of England. Of course, Queen Mary could have taken her personal crown - of 8 half arches - but chose not to. It held the Koh-i-nûr. Tricky!
Initially, was the new Imperial Crown of India designed to match Queen Mary's personal crown?
Edit: I have record saying Carringtons made the Imperial Crown of India for Garrards. I suppose Garrards were already rather busy with orders from a certain royal lady?.