If the second front was held up what effect would that have on the Eastern front?
Some, especially once the Germans knew that the allies wouldn't be coming, which is a somewhat different point from the allies knowing their not coming. However probably too late to delay the Russians a full year. What you would need is a better run Germany, especially in the battles on the eastern front or at home, to hold the Red Army back that much longer.
Also I have seen comments, think it was by Eisenhower, that if the V1 campaign had been allowed to continue Britain may not have been able to sustain an invasion. Not sure how accurate that was however.
The British economy would have sustained another year of war as it was basically part of the US war economy. British manpower would have held up without the OTL western front losses but would be nearing the end of its tether.
One question, if the allies don't land in 44, is what is Stalin's reaction. He was always demanding a 2nd and then 3rd front, even when he know it was impossible. Will he decide that, with the Soviets also suffering serious manpower shortages, its worthwhile trying to negotiate with the Nazis. After Baglation [sp] and the destruction of Army Group Central you might have seen some elements in the Nazis and army willing to overthrow Hitler to get a peace in the east.
Steve