re: Would the Allies have won WWII without America getting involved?

No. The British Empire would have gone bankrupt and been forced to accept a negotiated peace. The war between Germany, and its allies, against the Soviet Union would have been quite prolonged. The Germans would have had more men and material at their disposal, and the Soviets would not have had the logistical network that American lend lease supplies afforded them. I am not certain that this combination would have been decisive as the Germans also had two major flaws to their war effort, but I think the Soviets would have sought a negotiated peace. The bigger question is would Hitler have accepted anything less than unconditional surrender?

The first was Nazi policies with respect to the populations of occupied territories. Treating the local inhabitants as untermensch required significant resources to control the population. A more liberal approach, and the Germans would have found a more cooperative population. There were more than enough people who would have gladly welcomed the Germans as liberators, but the Germans behaved as conquerors in accordance with Nazi ideology. The second was Hitler's role in German military operations. His ideas were occasionally brilliant, but they were more often disastrous for the German military. He had some of the best military minds of the generation available, but he did not allow them to use their talents.

The outcome of the war between Germany and the Soviet Union would have been influenced by the terms of any peace between the Germans and the British. The Germans would most likely have gotten concessions that opened the oil fields of the Middle East to them via the Suez Canal and Italian ports. This would have altered the conditions that influenced German strategic planning in 1942 that led to the disaster at Stalingrad. Who knows what would have happened without that defeat for the Germans.

Russia would have been able to take over Germany without us. We just sped up this process with DDay opening up a two front war and supplying the allies for years. Germans were spread too thin. Now Japan, who knows. That would have been interesting.