On paper, at least, India under the Raj wasn’t the single nation “India” as we know it today, but a massively fractured series of kingdoms and micro-nations. (Think of the Warring States period in China, or Italy until the late 19th century — but cloned many times over.) Each was (in theory) independent, though deeply linked with the others through trade and treaty. Each (in theory) had its own arrangements with the British. In practice they were vassal states to a virtual vassal state (“India”) of Britain.
In ethical terms, there is a difference between taking advantage of a period of unrest to loot art objects, and taking things with the permission of whomever is in charge of the place where the artifacts are located. (In some cases, like Boston’s Japanese art collection, the items were literally being discarded during a period of unrest, and would no longer exist if some foreigner hadn’t taken a shine to them. VERY tricky.) Obviously leaders change, and by the standards of democracy virtually no leader from the past would now be considered “legitimate” — but that’s applying modern ethics to the past. Modern ethics are a modern technology.
Indians are wonderfully legalistic, and I’d be a little disappointed if they didn’t try to make a case for having the items returned. But Indians have a bad habit of building a convoluted case and then BELIEVING it too. I’m afraid that what this probably comes down to is nationalism, and that’s something that I, personally, have no truck with.