I think you will find that the diamond was a gift to Queen Victoria as opposed to it being stolen or pillaged (as was usual in past centuries). As such, I believe it's ownership has passed down through the hands of British monarchs and now - I suppose - it passes into the personal possession of King Charles. He might decide (as I would) to return it - as a gift - to its rightful owner, if such a rightful owner could be identified. If that were not the case, then he could (as I would) return it to the Indian Government as a gift to their people to be on display in an appropriate museum in perpetuity.
IF Charles - representing the Crown - wanted/wished to return the Koh-I-Noor to India, then nothing and nobody could stop him because he has absolute power. However, whilst his hereditary position as monarch historically precludes him from exercising this absolute power, it doesn't stop him from exercising his VERY persuasive powers which he has in abundance. If he wanted it to happen - then it would, but you'll see flying pigs first.