How did Regency System set up in West Japan
On August 6th, 1945 of the SF timeline, Prince Regent Prince Takamatsu Nobuhito have declared to change to name of Hiroshima as Saikyo(西京), literal meaning western capital in Japanese. Imperal family will formally stay in Saikyo from this point. He also declared an update the section of Regency of Imperial Household Law, amendment declared "When the state is under division, foreign occupation or the emperor isn't capable to rule himself due to underage or other circumstances, his closely related Prince will act in imperial power as Regent. The power of Regency will in act until nation reunite as one. Prince Regent will rule the nation accordingly though National Diet of Japan and cabinet meetings, Prince Regent will make judgment if failed to have Consensus decision-making from National Diet or cabinet"
Where did he gain these power and inflence from? It can be dated from the time back in civil war.
After coup plotters' attempted assassination against Tojo when wrong and caused the death of emperor Hirohito, they realized civil war with Tojo was inevitable. Tojo's predecessor, Fumimaro Konoe(近衞 文麿), angered by the assassination, declared his endorsement to Tojo, which brings entirety of Imperial Rule Assistance Association(大政翼贊會) on Tojo's back.
Coup plotters, mainly naval high commanders like Isoroku Yamamoto, led his navy escorted royal family to Hiroshima, where a coronation ceremony was held for Crown Prince Akihito on board of Yamato with era name Heisei (平成). His uncle Prince Takamatsu Nobuhito(高松宮宣仁親王), who is one of the main plotters, were also named to be the Prince Regent for the young emperor Heisei. Meanwhile, Tojo administration after regain control of Tokyo, recognized Akihito as new the emperor, but maintained its claim on "emperor was held hostage by rebel", declare the era name as Seika (正化). Prince Yasuhiko Asaka(朝香宮鳩彦王) was named as Prince Regent for young emperor Seika
Under Prince Yasuhiko Asaka's name, Tojo cabinet passed the law of conscription along with the establish Volunteer Fighting Corps (国民義勇戦闘隊). They were preparing mobilize for final war on homeland 本土決戦 against Allies forces and rebels, and many of them were only trained with Spiked Bamboo. Taiwan under Japanese rule, declared to follow the exact same policy.
After coup plotters settled the emperor and royal family inside Hiroshima Castle, Prince Takamatsu Nobuhito quickly sent a request to Allies powers, asking for peace and assistance from Allies at almost every cost, and all Allies postwar demands. After a week, they have able to won over control of army in Korea peninsula, mainland China and Indochina from their personal ties.
While in Philippine, Shizuichi Tanaka (田中 静壱) after crushed the Tojo supporters, declared Philippine and 14th army surrendered to his personal friend Douglas MacArthur. Hitoshi Imamura(今村 均) also led his troop in Rabaul surrendered to Australian forces after he heard about what happen back in Japan.
Southern Expeditionary Army Group across the rest of Southeast, except for Sumatra, declare to fight against rebel inside and enemy outside until the last breath.
Due to the lack of influence and prestige, Tojo have to concede power to his Prince Regent and his predecessor in order to keep his system function. When the Heisei side heard the news, they had to follow up because they realize, war of the legitimacy needs to have its leader, and their leader is Prince Takamatsu Nobuhito. And this position was strength after Allied power supports Heisei side, which gave Allied to have access of bases on Ryukyu Islands and landing on Japan soil without resistance.
Since then, US have back the regency system in west Japan during and post war for its interest in East Asia, never did they know at that time, the system has lasted more than four decades.