Short answers, pulled from the story.
Shah Jahan III was Mirza Muhi-ul-Millat, the elder son of Muhammad Kam Bakhsh who was the youngest child born to Emperor Aurangzeb. His genealogical thread connected him directly to the great Aurangzeb through three generations.
Shah Jahan III reigned from December 1759 until sometime in 1760 when ministers acting for Shah Alam II stripped the throne from him. The reign lasted only months before the end came swiftly.
Delhi witnessed political intrigue during the winter of 1759 when Imad-ul-Mulk orchestrated events that would place a new ruler on the throne. Ministers and courtiers maneuvered behind closed doors to secure their power within the capital city.
Ministers chose Mirza Muhi-ul-Millat to serve as their puppet emperor because his selection stemmed from his royal ancestry rather than personal merit or military strength. No other claimant possessed such a clear descent from the imperial house.
The period between 1760 and 1772 saw him removed from all state affairs and he died in 1772 ending a life defined by political manipulation. He lived out his remaining years without imperial power or influence.