1.	What did Truman know about discussions between Stalin and Japanese leadership?
While Truman did not know the exact details of Stalin's discussions with Imperial Japanese leadership, Truman had hoped to get the Soviets to join in the U.S. war against Japan. The Soviet Union had drawn up detailed operations plan for a major landing, in return, Stalin expected Soviet control over certain territories annexed at the beginning of the war by Japan and Germany. However, Truman had refrained from informing the Soviet leader about the Manhattan Project, which had just successfully tested the world’s first atom bomb but knew that the new weapon strengthened his hand. 

And so, on Aug. 18, in order to prevent the scenario of the Soviet occupation of a large part of Japan post-war, Truman bluntly stopped Stalin's final go-ahead of a land invasion of Japan.  Stalin procrastinated, weighing the pros and cons, and two days before the planned Aug. 24 landing on Hokkaido, Stalin called off the operation.
what do you think would have happened if Stalin didn't call off the planned landing on Hokkaido?
It's impossible to know for certain what would have happened if Stalin hadn't called off the planned landing on Hokkaido. 

It's possible that the Soviet Union would have entered the war against Japan, leading to a quicker end to the conflict. However, it's also possible that this could have escalated the already tense relations between the United States and the Soviet Union and potentially even led to a wider conflict between the two superpowers.

Additionally, if the Soviet Union had occupied a large part of Japan post-war, it's likely that this would have had significant implications for the balance of power in East Asia and the post-war world order. It's possible that this could have led to a prolonged period of tension and instability in the region.

In hindsight though, it's clear that Truman's decision to use the atomic bomb played a significant role in bringing the war to a swift end and shaping the post-war world order.