Japan's socialist and communist organizations had been utterly decimated by Japanese internal security from militarist factions before the war. It is unlikely that they would have been successful in a takeover of Japan without foreign help. In addition, the Soviet Union simply did not have the capability at the time to intervene in Japan proper-doing so would have compromised their support of the Chinese Communists and Kim Il-Sung's regime in Korea.

What is likely to happen would be something similar to Chiang Kai-Shek's Nationalist regime in China post-war. Due to a US arms embargo and lack of any real aid, Chiang's government had a difficult time reclaiming many of the territories occupied by the Japanese and later the Communists. The warlords that allied with Chiang gained a great deal more autonomy due to Chiang's resources being deployed against the Communist base in Northern China.

Suppose the US elected to institute a Weimaresque regime in Japan. Without a ruling emperor or other preexisting Japanese institutions, as well as a solid US military occupation force, it is likely that crime would have been even more rampant after the war than it was under the military occupation, as devastated industries and a return of unemployed soldiers would naturally gravitated towards organized crime as a source of income and security. The end result would likely be something like Du Yuesheng's rule over Shanghai, where local crime would setup almost feudalistic mob rule in various population centers, with local paramilitary composed of ex-military personnel, all ruled extremely loosely by a weak central government. The end result would be almost a sort of new Sengoku period, a sort-of Balkanized Japan more susceptible to rivaling foreign influences.

Japan and Russia had been enemies long before World War II. The Russo-Japanese War resulted in Russia being humiliated by what many Russians viewed as an inferior people from a small island chain. At the time, Russia wanted the port access that the islands of Japan provided, as well as a place to populate in the eastern part of their territory.

Despite these feelings, Russia took heavy losses in World War I and World War II. Attempting to move the army that was left to eastern russia would have been impractical because of both geography and political land-grabbing going on in Europe.

My understanding of China is that they generally viewed their land as the best in the world and weren't that interested in colonization of Japan. This I will openly admit I know less about, but again my understanding is that Japan tended to be the aggressor with the goal of obtaining land.

So in short, in my opinion after having studied Russia and Japan pretty heavily, I don't believe Japan was in terrible danger from outside forces. The occupation of Japan was more likely to encourage a Pro-Western government to arise (which it did obviously) and prevent any would-be revolutionaries from taking hold.