Demyansk cauldron. The missed victory of the Red Army

This battle lasted a year and a half, from September 1941 to March 1943. It was under Demyansky that the large German group first fell into the “boiler” and the first elite composition of the SS division "Dead Head ”came to the“ boiler ”, but the Red Army lost hundreds of thousands of fighters and up to a thousand tanks here.
Why was the Demyansk bridgehead never eliminated, and the brilliant operation of our troops turned into a multi -month positional meat grinder?
through whose fault was the victory missed that could change the course of the war?
How did the Germans manage to survive in complete surroundings, establishing the supply of the Demyansk fortress by air, then to release the "boiler", breaking through and holding a 40-kilometer "corridor", and in a year in organizedly to remove their group from the blow?
Is it true that this “Pyrrhic victory” of the Wehrmacht eventually turned into a Stalingrad catastrophe that it was because of the huge losses of military transport aircraft near the Demyansk Luftwaffe that the Air Air bridge for the surrounded army of Paulus was unable to bring? ..
The first detailed study of the Battle of Demyansk gives exhaustive answers to all these questions