Unconditional means Japan couldn't condition their surrender and have to accept what terms, if any, we gave them.
Normally unconditional surrender means that there are no surrender terms whatsoever. The person surrendering might receive mercy. But they also might simply be killed or enslaved.
I'm not sure how much actual power the emperor had during the war. Certainly all power was vested in the emperor after the Meiji Restoration but how much he personally wielded and how much was exercised on his behalf by the government is an open question in my mind and heavily debated.
That was exactly the problem. The Japanese military was wielding the Emperor's power for him. Had we granted Hirohito unlimited dictatorial power, we would have been granting the Japanese military unlimited dictatorial power.
If you have sources I'd be interested in them.
The book "Racing the Enemy: Stalin, Truman, and the Surrender of Japan" by Tsuyoshi Hasegawa covers the nuances of Japan's conditional surrender offer. It's in the last few pages of "Chapter 5: The Atomic Bombs and Soviet Entry into the War", and the first few pages of "Chapter 6: Japan Accepts Unconditional Surrender".
I don't agree with the book's main conclusion (that the bombs were dropped with the goal of ending the war early enough to save Japan from Soviet invasion and occupation). Everyone always always overthinks the issue of motivation for dropping the bombs. Sure there might have been various side benefits to using the bombs, but ultimately we were in a terrible war and were just trying to do everything possible to get the war over with.
But despite not agreeing with the main conclusion, I think the book has lots of valuable data.
Japan was apparently making surrender overtures to the Soviets before we dropped the bomb. I've been meaning to read the communiques but as yet haven't had the time.
It wasn't what I would consider surrender. They were hoping to simply call off the war and have everyone just pack up and go home without Japan being defeated or occupied.
There wasn't much to the communiques. The main proposal would have been conveyed by Prince Konoye, and the Soviets refused to receive him. All the communications between Japan and Russia were variations of the Japanese ambassador pleading with the Soviets to allow Prince Konoye to come and talk to them while the Soviets continued to stonewall.
There were communications between the Japanese ambassador to Russia and Japan's foreign minister Togo. They mainly consisted of the ambassador expressing skepticism that this could achieve anything more than a guarantee for the emperor, and Togo (who agreed with that assessment, but was bound to carry out the broader policy of trying to end the war in a draw) telling him to shut up and pursue the broader strategy.
In either case I'm not sure that it matters. Whether the Japan wanted an all powerful emperor or not, the fact that we didn't specify his fate probably contributed to the continuation of the war.
Yes, but only for three days (and after the bombs had already been dropped).
Before August 10, Japan was pursuing their broader strategy with the Soviets. If we had clarified the Emperor's status before then, Japan still would have kept pursuing their wider goal with the Soviets.
However, clarification of the Emperor's status could have had the war end on August 10 instead of August 14.
What might have had a bigger impact on the end of the war would have been to let Japan know somehow that the Soviets were planning to go to war with them no matter what. If Japan had realized that, they might not have kept pursuing contacts with the Soviets to the exclusion of all other avenues.
But that would have been hard to do because we didn't speak for the Soviets. There was a proposal to include the Soviets in the Potsdam Proclamation, which might have carried a warning of sorts, but it would have also allowed the Soviets to insist that the Potsdam Proclamation be much harsher towards Japan so that they would be less likely to surrender before the Soviets could enter the war.
In the end, war is ultimately a series of mistakes by both sides, with the side that makes the fewest mistakes emerging as the victor. It would be highly improbable for the US to have conducted the war so perfectly that the most optimal result possible would be achieved.