I am afraid that I don't have the elements to solve this contradictory puzzle.
On the Royal Collection's website, the following picture is dated 1912. Judging from Queen Mary's dress and jewelry, it is obviously from the same photosession Beth posted pictures of. So it could be coherent with the information gven by Roberts that the alterations made by Garrard in 1912 were aimed at adapting the two lesser Stars of Africa into the Durbar Tiara.
But of course, if these pictures were published as soon as May 1911, then the caption made by the Royal Collection and by Hugh Roberts (himself a former director of the Royal Collection) would be wrong.
Nevertheless, the celebrations of the Delhi Durbar (which is the first occasion on which the tiara was supposed to be seen) took place from 7th to 16th December 1911, with the Durbar itself occuring on 12th December 1911. Source: https://en.wikipedia.org/wiki/Delhi_Durbar#Durbar_of_1911 Therefore, I remain highly sceptical about the Durbar tiara being pictured in a paper in May 1911.
Beth, I am aware that the question may sound naive, but is the date of the London Illustrated News (20th May, 1911) fully reliable?
Anyway, I hope we will see this beautiful tiara soon again! It has been more than ten years now that this tiara has gone back to the vaults!!! It had unexpectedly popped up in 2005 for Camilla's first State banquet as Duchess of Cornwall, after more than half a century during which this tiara had remained away from public eyes... I hope that for the next State Opening of the Parliament, the Duchess of Cornwall will opt for the Durbar tiara, instead of her usual Honeycomb Tiara!