Sure, not in dispute. The point of the exercise is to recontextualize ownership of colonial spoils as the heritage of the colonized people, rather than the personal property of the monarchs involved, or the successor states of an empire. Indeed, I would broaden that to include all wealth plundered from the people, including by their own nobility. If the goal is to restore the diamond to the people who have had the greatest claim to it, it's not an impossible task to determine where their descendants currently live. If we seek to consider to simply restore the diamond to the last royal claimant (and their successor entity) before British hands entered the long relay of property ownership, I'm not sure much is accomplished.
The issue is colonial people in South and Central Asian history is weird because just about every South and Central Asian state (the Durranis, the Sikh Confedracy, the Sikh Empire, the Mughals, the British Raj, the Maratha Empire, the Delhi Sultanate, etc) was a federal system with castes, clans, and ethnic groups being given varying levels of self governance [0][1][2]. So when you say colonial people, does it belong the people of N state of the no longer existing country called X or M state of no longer existing country called Y. Successor States are an actual legal concept which remains a grey area to this day.
On top of that, both Pakistan and India made themselves successor states of princely states in the 1970s but it's still a murky gray area from a jurisprudence standpoint as cases are still being litigated, as the associated federal-local level deliniation is still lacking. Welcome to the legal world.
What does the legal self governance of any single historic state have to do with the question at hand? We have a fairly reasonable model for the people who constitute modern states, and how they were associated in historic empires. So even if the Persian Empire subjugated parts of modern day Armenia and the Persian Empire held the diamond for a while, do we seriously think the Armenian people have a serious transitive claim to the entirety of the diamond? A lot of the contenders you've raised can certainly be wilted down.
South Asian colonial history is no more complicated than anywhere else in the world. Nor at any point have I deferred to the legal inheritance of successor states.
Consider for example the Elgin marbles, claimed by the British from a section of the erstwhile Ottoman Empire. Do we seriously believe any of the successor states of the Ottoman Empire other than Greece should claim them? If the goal is to restore the marbles to their "home", the answer to where they should go is easy. Yes, the diamond's history is more complicated, but not impossible to figure out where it's "home" should be. I've offered just one possible model, based on a collective ownership model, and weighing it by the factors such as how long it rested in each place.
Sure, not in dispute. The point of the exercise is to recontextualize ownership of colonial spoils as the heritage of the colonized people, rather than the personal property of the monarchs involved, or the successor states of an empire. Indeed, I would broaden that to include all wealth plundered from the people, including by their own nobility. If the goal is to restore the diamond to the people who have had the greatest claim to it, it's not an impossible task to determine where their descendants currently live. If we seek to consider to simply restore the diamond to the last royal claimant (and their successor entity) before British hands entered the long relay of property ownership, I'm not sure much is accomplished.