There was no evidence that the people around

Hitler opposed Hitler's splitting plan but who could influence Hitler's decisions?  During the winter in 1941, Guderian argued about Hitler's "not a step back" order & was relieved.  Eventually, the Germans had to pull back before they could establish a relatively stable defensive line facing the Russian counterattack.  

The chief of army staff, Franz Halder, did not say a word about splitting Army Group South.  He did warn about 6th Army being cut off due to stretched front in September.  He was removed from the position by Hitler because of that advice.  I can list more generals' names being sacked by Hitler due to disagreements between the generals and Hitler. 

Back to the end of August 1942, the fourth Panzer Army led by Hoth had a chance to encircle the Russian 62nd and 64th Army.  They just needed he Sixth Army to seal it.  The Sixth Army was delayed by strong Russian resistance for 3 days.  Those two Russian Army escaped to Stalingrad in 3 days (most of them infantry units).  They later became the main force to fight the Sixth Army in Stalingrad. 

Operation Winter storm was risky but surprised the Russians.  The bridgehead was established on December 19th.  Manstein ordered the retreat on 23rd & the rescuing army returned to the German line on 24th.  It is tough to fight in severe weather conditions but the rescuing army (besides the 6th Panzer division the rest of units were as exhausted as the Sixth Army) could penetrate why couldn't the Sixth Army fight from the other end of encirclement?  The rescuing army received a lot of helps from Luftwaffe by the way.  Like the Russians, the Germans lacked truck.  Their main transport vehicles were horses and half-tracks.  

The Germans had around 60 tanks & around 250k soldiers (plus around 70k Romanians) at that time.  The supplies were not guaranteed and running low.  Blizzard was developing.  Part of Stalingrad was controlled by the enemy who received steady flow of supply.  How much chance could the Germans survive in the city if the possible reinforcement would not come till next spring?  In fact, the Sixth Army ran out the supply by the end of December & was constantly harassed by the Russian soldiers shared the same city.  How does it differ from camping in the wilds?   

Breaking out required certain amount of men power and materials.  It is why I suggested an organized concentrated force (5k maybe) breakout attempt.  Italian army had proven that they could fight nobody around the world (the only capable German ally was the Finns by the way).   Some units had to be sacrificed to cover the rest.  The key is that Luftwaffe continue to provide must-needed air supports for the break out.  A general needs to takes some chances in criticle situation.  A breakout at least gave a chance for the army to flee.  It looks bleak but not completely hopeless.  When the blizzard came, however, all hopes had gone. 

I was saying that Army Group A was not in immediate danger as long as the Germans controlled Rostove & Kerch peninsula.  In fact, Army Group A received supplies across the Black Sea.  If they were cut off, they were in a much better position to repel the offense. 

For Operation Bagration, you can't deny the mismatch between the Germans and the Russians.  I have stated before that Nazi Germany could last till 1945 thanks for the commanding skills of field generals.  During Operation Bagration, all German divisions were understrength.  Some divisions had the staff only!  After Operation Bagration, Guderian has considered to negotiate a German surrender but was rejected by Hitler and his Nazi fellows.  Guderian was soon relieved again with "health reason".   

One question, why do you keep comparing Russo-German War to the Chinese Civil War?  Soviet Unions had strong allies & Soviet Union alone out-produced Germany in every key war equipment.  The Chinese communist forces were basically on their own.  They were outmatched by KMT in either men power or war materials.  Two different scenario.