The Poles and the French are east of Smolensk? Then I do not think any Soviet attack can reach Warsaw at all. Sorry, but it is simply impossible. However, Army Group North can be cut off if the Soviets turn north east of Vilnius. Otherwise, the Red Army will have very long, narrow, extremely hard to keep corridor. Since the Allies control Baltic, they can supply Army Group North by sea. And German forces from south can cut off Soviet advancing forces.
The only way that offensive can succeed is to beat Poles and especially the French. Why especially them? To beat France psychologicaly, to shock French public opinion enough to get France out of the war and at least force the Allies to peace negotiations (and Stalin can forget about Ukraine, I think).
Now, what should the Allies do? Let's assume the Soviet attack crashes Polish-French lines and forces them far west. Do the Soviets have enough strength to engage Germans in north and south so they will not be able to help the centre? I have my doubts. As Allied advisor I would recommend Poles and French going back in fighting retreat, exposing Soviet flanks to German counter attacks. Army Group North shall keep its front with infantry and throw all its armour against Soviets' back and flank. The same in south, to create the biggest cauldron ever.
The only way that offensive can succeed is to beat Poles and especially the French. Why especially them? To beat France psychologicaly, to shock French public opinion enough to get France out of the war and at least force the Allies to peace negotiations (and Stalin can forget about Ukraine, I think).
Now, what should the Allies do? Let's assume the Soviet attack crashes Polish-French lines and forces them far west. Do the Soviets have enough strength to engage Germans in north and south so they will not be able to help the centre? I have my doubts. As Allied advisor I would recommend Poles and French going back in fighting retreat, exposing Soviet flanks to German counter attacks. Army Group North shall keep its front with infantry and throw all its armour against Soviets' back and flank. The same in south, to create the biggest cauldron ever.