For me the main risk you face is a Japanese juggernaut starting in Mongolia and then taking each base on the railway. By the way with the river crossing = shock attack rule, attacking USSR is harder becasue you alwyas have to cross a river to do this, and can't hope that a huge army will do that in one day. AFAIK all AAR with an invasion of Siberia were done before this rule.
At this stage of the war, you should have all troops prepared 100% and all bases with fort 9, and also a good number of troops in the open.
For me the key of the campain will be to win enough time to enable Allied bombers (both Soviet and US coming from Alaska) to bomb Manchouoko and a part of Japan to rubble...
In red, I have fixed the part of Siberia that you should hold at any cost. Iman is a mountain hex, and is excellent to stop a juggernaut coming south on the railway. Vladivostok has a lot of troops preparing for it and so should be easier to defend. Uglaharmensk is an airfield that can't be pounded to dust by BB and so should be you main air base. The problem is that it may be attacked while bypassing Vladisvostok. So maybe Iman should be you main air base too.
All red arrows penetrating Japanese territory should be seen as probes to test Japanese defences... The only change to stop a juggernaut (except maybe in Iman with a defensive juggernaut of yours) is to divide it, and to do it you must threaten the industrial cities of Manchuoko. You have a lot of tank units, you can use them in the open terrain east of Harbin.
Once your troops will be activated, you will have a month to organize. Move units all along the border to be able to see the concentration of Japanese troops. Move air force support units to where you will probably need them. Change preparation orders from unist in town you won't defend to Iman, or Vladivostok (the two points you don't want to lose).
A problem I had with my Russia game vs Moses was that most of the times Soviet units retreat leaving behind huge piles of supplies that will be really needed. So use the preparation month to set all bomber units to carry supplies from places you won't defend to place you will hold at all costs (remember ? Iman and Vladivostok).
On the side, I will prepare American and Canadian reinforcements in Alaska. Holding the small Soviet base isolated between the Aleutians and the Kuriles will be nice to ferry troops and aircraft, so as soon as the war is engaged use an air bridge to bring at least an US RCT here. Then you may bring US engineers or aviation support to Siberia, whatever is the most useful for you.
By the way what is the supply situation in Siberia. Do you have huge supplies stocks in the main cities, or are you more or less starving? If the first is true, bring units to cities you won't defend and turn replacement on there to use these supplies (if you have replacement off by default).
Edited: I forgot the edited map
