I was wondering if the front in Europe could've been won without U.S. help, and if the Soviet Union played a larger, more fundamental role during the war.
No, the German army at that point was no were near in a state to effectively defeat or even defend against the Soviet Army, and at that point, getting very close to stumbling on its back foot. Not only did the Red Army have an overwhelming majority in terms of equipment and manpower, but at this point was on par with the Germans in terms of effective leadership, courtesy of the experience and knowledge brought on by 3 years of fighting and the increase in operational freedom given to the STAVKA as well as the experience gained by the lower ranks of the officer corp. The intention of the German army during this time was no longer to overwhelm and destroy the Soviet Army, but to disrupt the process of Russia building up their forces. Model's explanation to his staff when he arrived on July 1st of 1943, during the planing of Citadel, in describing the front wide strategy, was "They (the Soviets) are using their time replenishing for winter. We must not allow that or there will be crisis....".
A grand strategy calling disruption is not the strategy of a army on the verge of victory. The numbers speak for themselves. The Soviet Army was fielding at the time 6.5 million troops, while the Ostheer, was fielding 3.5 million, which at the time represented the VAST majority of their standing army. Not only that, but most of their divisions were severely undermanned. Finally, the Soviet Union was producing immense amount of equipment, while the Germans, short on supplies and wasting materials on their "quality war", found themselves outstripped by Soviet industry. Soviet production of the T-34 and its variants numbered 14,000 tanks produced in 1943, ,while the Germans produced roughly 7,000 pzkpfw III and IV during the same period. The loss of nearly 1,000 German tanks and their veteran crews during Operation Citadel was simply something the Germans could not afford at the time, and those losses were not quickly replaceable. Following Kursk, the German Army was not even capable of mounting "Disruptive offensives" anymore. Overlord did much to hasten the end of the war, but in regards to changing the course of the war, no. The German army would have to somehow destroy an Army twice its size while incurring minimal losses, using their largely horse drawn logistic system to some how carry themselves back to Moscow, using an army undermanned, under-equipped, and largely devoid of any method of achieving decisive victory in the perceivable future.
Sources: Keegan, John.* The Second World War.* New York: Viking, 1990.
Weinberg, Gerhard L.* A World at Arms: A Global History of World War II*. Cambridge: Cambridge University Press, 1994.
Here is the story of the war told in German divisions by front by month.
After the summer of 1941, the Soviet's faced a larger number of German divisions than the Western Allies. However the decisive moment in the war was the Summer of 1944 when the British and American landings combined with a Soviet offensive to badly degrade the total number of German divisions.
Counterfactuals about whether the Soviet Operation Bagration would have succeeded without a Normandy landing are fruitless, in the way most counterfactuals are. However it's clear that the simultaneous pressure from both fronts resulted in an immediate collapse and massive losses that made an eventual German defeat inevitable.