While working on the history and evolution of the Durbar tiara, I have noticed two details, that lead me to think that the Durbar tiara is not at the disposal of the Duchess of Cornwall anymore:
1) When the Duchess of Cornwall wore the Durbar tiara on 25th October 2005 for the State banquet of the Norwegian State visit, the base of the tiara was wrapped with a pale yellow fabric (matching the hair colour of the Duchess), as it is usual to make tiaras more comfortable to wear:
But when the Durbar tiara was displayed in 2012 (first pictured for Hugh Roberts' The Queen's Diamonds, then displayed at Buckingham Palace's Diamond Jubilee summer exhibition), this fabric wrapping had been removed, so that the metallic base of the tiara frame was visible:
Why removing this fabric wrapping if the tiara was at Camilla's disposal and could possibly be worn again by her? All the other tiaras displayed at the Diamond Jubilee exhibition in 2012 at Buckingham Palace (the Girls of Great-Britain and Ireland Tiara, Queen Alexandra's Kokoshnik...) still had their base wrapped with fabric, matching with the Queen's grey hair. Only the Durbar tiara had the wrapping removed, which leads me to think that it was not intended to be worn again by the Duchess of Cornwall.
2) This impression was reinforced by the slight, but probably careful, differences of wording by Hugh Roberts in The Queen's Diamonds: - about the Greville Honeycomb tiara: the tiara "was inherited by The Queen in 2002 and has since been loaned to The Duchess of Cornwall" (page 240) - about the Teck Crescent tiara: the tiara "was inherited by The Queen in 2002 and has since been loaned to The Duchess of Cornwall" (page 122) - about the Greville Festoon necklace: the necklace "was inherited by The Queen in 2002 and has since been loaned to The Duchess of Cornwall" (page 250) - about Queen(Mother) Elizabeth's Coronation riviere: the riviere "was inherited by The Queen in 2002 and has since been loaned to The Duchess of Cornwall" (page 224) - but for the Delhi Durbar tiara, the choice of words is different: the tiara "remained with [Queen Elizabeth the Queen Mother] until her death in 2002. In 2005, it was loaned by The Queen to The Duchess of Cornwall" (page 178).
I guess that these differences are not just a mere coincidence. I understand them as if the Durbar tiara was only a one-shot loan from the Queen to her new daughter-in-law for the Norwegian State banquet in October 2005 (maybe for her to "make a statement" for her first State banquet as the wife of the Prince of Wales, a few months after their wedding?), with the tiara reverting back to the Queen's vaults soon after the banquet - contrary to the other jewels mentioned above, which seem to be permanent lifetime loans.
So, I think we should not expect too much Camilla to wear the Durbar tiara again (at least during Queen Elizabeth II's lifetime), and we should not include this tiara in the collection of jewels at her disposal...
Anyway, I hope the Duchess of Cornwall will give us more surprises in her choice of tiaras in the next months, instead of always sticking to the Greville Honeycomb tiara - why not the Teck Crescent tiara for the next State Opening of the Parliament?