From: Vermont, USA
Is a German breakthrough to southern Stalingrad possible in the game?
No, it really wasn't possible historically given the Soviet deployments so it's not possible here. The German intelligence on what they were facing was not great and once the 2nd Guards Army started deploying in front of them near the Myshkova River, they had no chance. On top of that, Little Saturn was launched against Italian 8th Army, which necessitated the withdrawal of the 6th Panzer (the strongest of the three Panzer divisions assigned to WinterStorm) along with 48th Panzer Korps (which was unable to help earlier due to problems of its own along the Chir). In short, the Germans were in a very bad situation and I'm not sure even Thunderclap could have helped them, had Hitler authorized it. If somehow the 48th and 57th worked together and Thunderclap happened before the 2nd Guards Army was redeployed, then perhaps something could have come of it in terms of at least some of 6th Armee reaching friendly lines, but that's still debatable.
With all that said, the Germans have a very interesting campaign to fight and succeeding in all battles will put you a bit ahead of their historical progress, but the real goal is to preserve and improve your forces - winning at the lowest possible cost.
It's also possible for someone (including us) to design an alternate campaign in the future that explores some more hypotheticals or unlikely situations as well as extending the existing campaign to allow a drive further on as long as the player's forces hold out. Historically though, there were very severe limits on what the Germans could hope to accomplish because of both the operational and strategic situations.