Was the Nagasaki bombing (or Hiroshima, for that matter) really necessary

Today is the 75th anniversary of the dropping of the world's second nuclear bomb, on Nagasaki, Japan, on August 9th 1945. The first was dropped three days earlier, on August 6th 1945, on Hiroshima. The first one killed between 90,000 and 146,000 people, and the second between 39,000 and 80,000.
I have often wondered whether, even if the first bomb was justifiable as a "shock and awe" tactic that ultimately shortened the war and saved many lives overall (and that's a big if), was the second one necessary at all?
An interview with q Japanese historian and the daughter of two hibakusha (survivor of the nuxlear blasts) has made me seriously doubt that, and the argument is not the usual one about the horrors of war and the immorality of death on such a huge scale, but a strategic argument about the military history of the times.
It's no secret that Japan officially surrendered on August 15th 1945, just 6 days after the Nagasaki bombing, and Emperor Hirohito apparently came to the decision as early as August 10th, the day after Nagasaki, so it is easy to conclude that the surrender was a direct result and consequence of the bombing(s). What I hadn't realized, though, is that, at midnight on August 8th, i.e. between the two bombings, and just hours before the second bomb was dropped, the Soviet Union declared war on Japan.
Many (non-American) historians feel that the entry of Russia into the fray was more important in the Japanese decision to surrender than the American bombs. After the Hiroshima bomb, Japanese leaders had already started seeking Soviet mediation in talks with the US aimed at bringing hostilities to a close, and when the Soviets declared war, that was seen as the end of any possible mediation process, and the final nail in the coffin of Japan's war effort. The Nagasaki bomb, and maybe also the Hiroshima one, it is argued, was therefore redundant, and more of a war crime than a saving grace.
It's not clear to me to what extent the Americans were aware of the Soviet situation, and we should probably remember that communications were not as efficient in those days as they are now, but hell, they had phones didn't they? It potentially puts a while new complection on the final events of the war, doesn't it? But I suppose we will never really know.