As Sloniksp pointed out, once encircled, 6th Army is doomed. You have to maintain an open route and not allow a complete encirclement. There are very, very few cases on the Eastern Front of large German pockets being able to fight their way out successfully. The few that do manage were always ones with a high degree of mechanization and mobility to begin with. 6th Army lacks this.
The Luftwaffe is not going to be able to stop the Soviets or prevent an encirclement. In my version they are there only to provide a means of slowing the Soviets and making their operations more difficult. The ground forces have to hold the pocket open and allow units to escape.