“...After Stalingrad, the defeat of Germany was certain, just maybe not by May 1945...”
After Stalingrad, the Germans had a remarkable victory in the retaking of Kharkov. There were certainly chances for a longer stalemate on the Eastern Front but Hitler kept stripping away divisions to gamble in other theatres like Italy.
How much of the Eastern Front was Kharkov? Did you pay attention to the rest of it?
The Germans were only able to fight at Kharkov and later at Kursk by shortening the front giving up the Rzhev Salient and freeing up divisions from there.