Short answers, pulled from the story.
Prince Higashikuni served as prime minister for 54 days, from the 17th of August 1945 to the 9th of October 1945. He is Japan's shortest-serving prime minister and the only member of the Japanese imperial family ever to head a cabinet.
Higashikuni resigned on the 9th of October 1945 in opposition to an order from Allied occupation authorities to abolish the 1925 Peace Preservation Law. That law had been enacted largely to suppress the spread of Communism in Japan.
On the 17th of October 1947, the Allied occupation abolished the imperial branch families, stripping Higashikuni of his title and most of his wealth. As a private citizen he operated several unsuccessful retail businesses, founded a Zen Buddhism-based sect called the Higashikuni-kyo (which was subsequently banned by occupation authorities), and published his wartime journals and memoirs.
Emperor Showa and Lord Privy Seal Kido Koichi judged it unwise to place a member of the imperial family in the prime ministership when war seemed likely, fearing that failure in war would damage the imperial house. General Hideki Tojo was appointed instead, two days later. Six weeks after that decision, Japan attacked Pearl Harbor.
Higashikuni died of heart failure in Tokyo on the 20th of January 1990 at the age of 102. He is the longest-lived of Japan's prime ministers and, from the 14th of May 1988 until his death, was the world's oldest living former head of government.
Higashikuni commanded the Imperial Japanese Army Air Service from 1937 to 1938 and the IJA 2nd Army in China from 1938 to 1939. A memo discovered by historian Yoshiaki Yoshimi showed that he authorized the use of poison gas against Chinese forces on the 16th of August 1938.