Short answers, pulled from the story.
Shah Jahan III was Mirza Muhi-ul-Millat, the son of Muhi us-Sunnat and the grandson of Muhammad Kam Bakhsh, the youngest son of the emperor Aurangzeb. His direct bloodline placed him within the Mughal dynasty, yet he served as a figurehead rather than a true ruler during his brief reign.
Shah Jahan III sat upon the Peacock Throne from December 1759 until his deposition in 1760. His reign lasted less than a year before he was removed from power by the Mughal ministers who had previously elevated him.
Imad-ul-Mulk, the vizier, controlled the state from behind the throne while Shah Jahan III remained a passive observer. The minister manipulated the young prince to maintain the illusion of imperial continuity while holding actual power in Delhi.
Shah Jahan III was imprisoned in the Red Fort and stripped of his titles after the ministers restored Shah Alam II to the throne. He died in 1772, over a decade after his brief tenure, with the details of his final years lost to history.
The reign ended because Shah Alam II returned to challenge the puppet regime and the ministers turned against Imad-ul-Mulk. The conflict was a civil war within the crumbling empire where sovereignty was contested by every faction from the Marathas to the Rohillas.