Japan didn't do terribly well against Russia in the 1930s or the 1940s. Any Japanese attack into Russia would have been the equivalent of the Italian attacks into France in 1940 - their only actual achievement was keeping the border forces busy (forces that would not have been moved elsewhere anyway).
Stalin in general was a marvellous politician but an awful human being.
WW2 was the best thing to happen for Soviet Russia, as the country was still rather fragmentary with the various areas (Ukraine, for example) still wanting independence, and the people as a whole still reeling from the harsh crackdowns that followed the revolution (which promised so much freedom).
The German invasion gifted Stalin with a common enemy for his peoples, and also gave him the excuse of 'it is necessary' to demand and get horrendous sacrifice, to further tighten his control, and to build up his personality cult.
Yes, Russia in the 1950s was a different place to Russia in the 1910s, but that was true of every country and change had begun in Russia before the Revolutions. It is likely that Russia would have been better off without Stalin - certainly the periodic purges did not help (check out how often scientific and/or designer types were purged and then rehabilitated once it became clear that they were actually needed).
To the charge that Stalin did not discriminate in his bloodshed...
1) Purges by political views (throughout)
2) Purges of Kulaks in the 1920s
3) Purge of the Orthodox Church in the 1920s-30s
4) Purge of Muslims in the 1930s
I'm sure there are more. It is also often stated that Stalin was planning to purge Jews before he died.