For the question "Were the Germans too spread out?" alone, I can offer some insight. One of the major problems Hitler faced in running the Nazi war machine was the dilemma of supplying enough oil, especially along the massive supply lines he was dealing with. As this article from 1942 points out, oil fields to the East were of primary importance, and were one of many factors motivating Operation Barbarossa. The Soviet scorched earth policy, as well as the destructiveness of the Luftwaffe's own bombing campaigns, left what refineries the Nazis did take in the Eastern Front in an unusable state.

Vast portions of Europe lacked sufficient oil infrastructure to meet the demands Hitler was pressing on them, so as the Nazis annexed and occupied more and more of western and central Europe, their oil deficit grew deeper and deeper despite the seizure of oil stockpiles (anecdotal comparison: similar to what hindered the Byzantines apropos of grain when they attempted 'reconquests' of former Roman lands).

This collection of testimonies by German figures on why they lost has some thoughts on the oil subject, such as:

"In my opinion, it was the Allied bombing of our oil industries that had the greatest effect on the German war potential. Even our supplies for training new airmen were severely curtailed--we had plenty of planes from the autumn of 1944 on, and there were enough pilots up to the end of that year, but lack of petrol didn't permit the expansion of proper training to the air force as a whole."

Although I cannot find a source for this so take it cum grano salis, one amusing and illustrative story regarding Nazi fuel problems follows that the Nazis were staging a grand parade of tanks through the center of a newly annexed capital, but ended up running out of gas shortly before they were due to arrive. Only a last minute refuel from local gas reserves allowed them to continue. I will continue to search for the source of this story.

Concluding: in sole response to your inquiry about Nazi overextension, fuel woes were of gravest concern even before the United States entered the war, keeping the Nazis in check on top of the other factors of manpower and spread forces. This recap breaks it down most scientifically, providing exact figures on barrel reserves, importation, and consumption throughout the war. Ignore the first paragraph about the modern US.

It is true that the United States engaged in less ground combat than the Soviet Union. However, that does not mean its contributions were any less important to winning the war. While German victory was never more than questionable on December 6, 1941, the entry of the US into the war assured it's defeat.

First of all, the mere presence of the US in the war made the threat of an invasion of mainland Europe a dangerous reality for the Germans, and forced them to garrison substantial numbers of men and equipment in France to defend against it. These were well equipped, well trained units that could have been fighting the Russians, but instead had to sit in France and just wait for the Allied landings.

Second, as was mentioned elsewhere, the Lend/Lease program was vital to keeping the Soviets on their feet and in the war. All those trucks, clothes, tools, spare parts, etc. gave the Red Army a huge logistic advantage over the Wehrmacht that can't be ignored.

Third, the Strategic Bomber Offensive played a substantial role in hurting German industry.

Lastly, one of the reasons it was so vital that a western front be opened up was because there was a real fear on the part of the Western Allies that the Soviets would be worn out fighting, and seek a separate peace once they has recovered all occupied Soviet territory. Without the US, the UK and allies didn't have the men or materiel to mount a cross-channel invasion and force the Germans to deal with two fronts.

So in summary, while the Soviets did more fighting on the ground, the American contribution was just as important.