The Royal Collection Trust should basically offer a monetary sum or estimate on stones/ pieces taken from other countries that are continuing to cause and bring about differing levels of hate and requests for retrieval of same.
Or they could simply go the route of returning these pieces and acquiring new pieces for the collection. They can raise the money to do this by simply though their commercialization of royal estates as they already do. And almost everyday they are astounding new pieces coming onto the market and some still in the ground yet to be discovered.
This would solve future problems for such persons as King William and his son George if they do take the throne. And Prince William has been understood to have been more prone to getting rid of stuff like all ivory in the royal collection so maybe his reign may see the eventual return of such pieces like the Lahore diamond and others.
Only my thoughts, but thanks Beth for the story, very thought provoking and they are simple remedies to solve it all.