1 Not very, it's one thing to conduct a rally on false information and another to have a deep hit that went so fast that a gap was created.
Well, and from Belarus to Kiev is closer than from Silesia to Warsaw. Even if it were possible to move the Poznañ Army and block the march from the main axis of attack, there are still forces from the north that threaten the city.
2 No, the Soviets are going down anyway. Either from the war of the anti-communist coalition after the fall of Germany or by themselves but much faster because the party does not have the myth of the Great Patriotic War by which this system will breathe away faster.
Why exactly would there be a war against the Soviet Union after Germany is defeated in this TL?