My main question concerns Germany and Japan. Did Japan actually treat their POWs the worst?
According to the findings of the Tokyo Tribunal, the death rate among POWs from Asian countries, held by Japan was 27.1%.[40] The death rate of Chinese POWs was much higher because—under a directive ratified on August 5, 1937 by Emperor Hirohito—the constraints of international law on treatment of those prisoners was removed.[41] Only 56 Chinese POWs were released after the surrender of Japan.[42] After March 20, 1943, the Japanese Navy was under orders to execute all prisoners taken at sea.[43] Around 1,536 U.S. civilians were killed or otherwise died of abuse and mistreatment in Japanese internment camps in the Far East; by comparison, 883 U.S. civilians died in German internment camps in Europe.[44]
3.3 of the 5.7 million Soviet POWs captured by the German Army during WWII died in captivity; either from starvation, exposure, summary execution, being gassed at in the camps, worked to death in a labour camp, or as a result of medical experimentation.
As a German soldier in Soviet captivity, your chances of survival were far better than those of your Soviet counterparts in German hands. Also consider that many returning soviet POWs were executed or jailed by the state for having surrendered to the enemy, or for having collaborated with the enemy (Ost Battailonen, Hiwis, etc).
The Japanese treatment of POWs was pretty uniform, ie very poor for all, but the Germans treated the Soviet POWs, their greatest number of POWs, with utter brutality.