Soviet forces lacked independent lift for an invasion of Japan

Could the Soviet Union mount an amphibious operation against Japan in 1945 as part of Operation Downfall?

Not with its own lift; Soviet forces would need British or American shipping, offered only if their participation became necessary.

Available troops were only part of the problem because the Soviet Union lacked the shipping needed to put them ashore. It could not independently mount the proposed amphibious operation against Japan. British or American ships would have to transport the force, and the Allies were prepared to consider that only if the troops were needed. Political mistrust would also shape any such arrangement.