What is my 2cents worth well here goes

Soviets did not under estimate the size of the "Stalingrad Pocket" they knew who, what and how many that were trapped in Stalingrad.

In the event of encirclement it is known that the best time to breakout of an encirclement is just after the encirclement is created as the Soviet forces would naturally be bouyant and have their guard down, and it is this time to strike out and effect a rolling offensive.

One thing that has not raised its ugly head and that Paulus lied to Manstein on the level of mobility, this was to downplay the 6th Army and part of 4th Panzer Army's effectiveness to effect a breakout. But had Paulus had gotten an order to evacuate after a lucid moment by Hitler, who orders the withdrawal of Stalingrad then the 6th Army and 4th Panzer Army (Part) would have succeeded in breaking out.

Just my 2cents worth.