There was an American who served in the Soviet Army and American Army during WWII. Joseph R. Beyrle , He was a paratrooper who was captured by Germans. He escaped a prisoner of war camp and found and served with a Soviet tank battalion.

It was also decided during the Yalta Conference the Soviet Union would join the war against Japan a couple of months after the fall of Germany

There were numerous times. In September 1941 the British sent a Hurricane unit to help defend the Soviet port of Murmansk (No. 151 Wing). Also in 1941 Stalin requested in a letter that the British deploy "about 40 divisions" (roughly 80% of their army) to the southern flank through Iran. This was obviously not done. It's also pretty unlikely that anything like it would have been attempted even if Normandy had failed. By 1944, the Allies were firmly established in Italy and would probably have sent the survivors there.

Later there were a number of shuttle bombing missions over Germany, where the bombers would not turn around and return but instead continue on into Soviet-held areas, land, refuel and rearm and then return to England. The Soviets also operated a unit of fighters with French pilots (the Normandie-Niemen Regiment).