Your TL's are always top class and this is no exception, although I can't really see what the Soviet strategy is here. They can (rightfully) deride the Japanese for their poor grasp of strategy but at the same time their entering into a relationship with an ally of convenience with an all too dubious paper trail. Even if they don't know the Japanese are having their mail read it would seem rather too risky for the potential reward.
That's the risk. The United States and britain *are* reading a lot of Japanese mail, especially diplomatic.
The Soviets had penetrated both governments to some degree, though it is less clear to me how much of MAGIC or PURPLE they knew about.