I’m liking the ideas here :-)
In most of my games the Soviets make a fighting retreat from the border until they can link up with offensive units (the tank and mech from moscow and the tank and mech from stalingrad, planes from moscow as well) moving east to Counter the Japanese thrust, when or if it comes.
I think there are 2 ways to address the issues here, one is simple and cosmetic, the other is mechanical and more complex.The cosmetic fix would be to change the setup giving the Soviets a few less infantry, maybe swaping out several infantry for a tank or artillery but reducing the huge number of Soviet Infantry. Moving some of Japans considerable air forces to other provinces, like central China or formosa, so Japan could still attack the Soviets if they wanted to, just not right away, at least not at full strenght anyway. Also, house rules are another cosmetic change, like saying units capable of blitzing cannot do so in China OR siberia. Little things like this may effect a turns worth of play but not the deeper mechanicas of the game.
A deeper, mechanical fix would be to change the area in such a way as to allow the Soviets to respond to a Japanese attack. As it stands now, the Soviets have no real way to respond to a Japanese attack outside of slowly retreating offering up small groups of infantry to delay them. Allowing the Soviets to actually respond to a japanese would completely change this usually static theater and also allow to reduce garrisons on both sides. Say we took the the Yakut and increased its value to 2IPC (and to balance things we’ll reduce timguska to 0). Now the Soviets have a territory with in striking distance of the Japanese border that can house a new minor IC, should they choose to build one. This radically changes the entire theater as it not only give the Soviets a way to more effectively counter the Japanese, but it also gives the Japanese something to work towards capturing. This also allows for the reduction of garrisons on both sides, since the Soviets can now reenforce this region without having to march forces from moscow, there is no need to give them the 18 infantry in the region at the start of the game. the Soviet player can build up the defence of this region if they want to, so you could reduce their starting forces to 6infantry in Amur, and maybe only 3 in Buryatia and Sakha,or less. I think this would also give rise to reviving the Alpha+2 system where one power gets 12IPC if the other attacks it. Now, rather then just throwing down 2 tanks or 4 infantry in Moscow or Stalingrad, the Soviet player has the option to place a minor IC in Yakut and actaully challange the Japanese invasion BEFORE it gets to Moscow. Also, the reduction in forces would give the Japanese player a bit more flexability with what he HAS to keep behing in Manchuria and Korea.
Just something to think about