How could the shock of losing WW2 lead to a shift in Japanese politics towards the kind of isolationism that was present before the 19. century(sakoku), and how isolated from global trade could this Japan be?
Yes, the pre-Perry period. I am aware a complete recreation of that is impossible, but could strict neutrality and isolationism be achieved?Are you talking about the pre-Perry days when the only contact any Japanese people had with the outside world was via a few trading posts on the peripherals of the archipelogo? If so, I'm kind of wondering why anyone in Japan would want to return to that.
Okay, I guess North Korea sort of pursued that policy, but in that case, a) the regime was imposed in the country by military force, and b) after the Korean War ceasefire, with the war itself still going on and an American-allied state right across the border, it was probably pretty easy to convince a lot of people that the slaughter was ready to start up again, if they didn't toe the line.
Those sort of factors don't seem readily apparent in postwar Japan. If the Soviets were to invade and impose an NK-style regime on the country, it would be somewhat awkward to whip up anti-US sentiment based around the wartime carnage, since the US had bombed Japan ostensibly to pursue the same broad war goals as the Soviets.
Best bet, Soviets invade Japan at the end of WWII, then the US turns against the Soviets and starts another war to get them out, thus leading to a Korea-style conflict on the archipelogo, with either the US or the USSR winning and backing a crazy nationalist government that fomets hatred of the other superpower.