Hrmm.. i dont really understand why this is a issue about realism. Why should it be impossible for the russians to develop the A-bomb or a even delivery system.
Russian inventions and designs in many fields far outclassed the other nations during the war. Necessity must really be the mother of all inventions.
How about the space program, a few years later? The russians where miles a head with their domestic rocketdesigns (how is that for a delivery system

). The US spaceprogram just consisted of the ideas of one Werner von Braun, that they (like many) had snatched from germany just before the trials at the end of the war.
Now a question about rockets/ballistc or guided missiles like the german V1-3 series, are they included somehow in the game? Because used right they could have had serious impact.