The only thing I would like to add is this: yes, stack Karelia as much as possible. Also, stack all your eastern units in Yakut to make a small wall to stop the Japanese for a few turns. Also your allies, US and UK, should be sending wave upon wave of reinforcements to Karelia as soon as possible. Then, when the chance presents itself, UK or US should try and take EE from the Germans. This blocks Germany from attacking Karelia with armour from the capital. Yes, Germany will re-take EE each turn, but by having the US and UK keep taking it back each time, Karelia is protected behind this “buffer zone” while Germany is prevented from hitting it full-force. This buys Russia some flexibility in its infantry placement; it can even send a few much-needed helpers down to the capital to fend off the Japanese.
Also, while the US and UK are busy sending waves of infantry to Karelia, have the US build at least one fighter every other turn. Send these fighters to Russia or Karelia. After a while, you will have a formidable defensive force of fighters in Russia/Karelia - a force that will rock Japan or Germany if they choose to attack it. Right now I am playing as the Allies, and after building up Russia with infantry from the US and UK, I have been sending US fighters over as well. I now have a total of 10 Allied fighters defending Karelia and Russia! This force will be deadly if my opponent is foolish enough to attack me!