My father in law was involved in the surrender negotiations - he was a Signal Corps officer. I can tell you from his first hand knowledge that your theory is completely off base.
Given the information available at the time -- not after the fact interviews of Japanese military or 20/20 hindsight -- there was not an alternative, reasonable or otherwise. The Americans were working from limited data, and the material that was developed from breaking the Japanese codes contradicted much of what the Japanese negotiators were saying at the time. After all, we were trying to negotiate with an actively hostile power.
(you realize I suppose that a Japanese fleet carrying weaponized biological agents - plague, anthrax, etc. - was actually en route to the west coast of the U.S. at the time of the surrender?)
Anecdotal appeals to authority are hopeless.
At the time, plenty of people saw that the unconditional surrender demand was unreasonable and prolonging the war. It has been issued in order to keep the wartime allies united, especially to prevent any last minute splits in dealing with Germany. Its application to Japan was required mostly by stubbornness and a desire of the pols involved to appear powerful and consistent to their own populace. Not exactly mortal considerations when in the balance with millions of lives.
It is ludicruous to pretend Japan was still any threat to the allied powers. It was necessary to finish the war, certainly. That is all.
The key issue was the political opposition to surrender within Japan. In case everyone forgot, the military staged a coup against the surrender even after the bombings - it didn't make *them* willing to surrender. It did make the emperor willing.
The Japanese had the delusional hope at the time that the Russians might remain neutral and help them negotiate a peace on terms better than unconditional surrender. That prop was knocked away when the Russians invaded Manchuria, in the same week as the bombings. Loss of that hope, from Russia's entry, plus an offer to keep the emperor but otherwise surrender unconditionally, might have been accepted.
Or it might not have been. But not to even offer it, even by back channels?
It is much harder to justify that. In fact it is impossible, in my opinion. Justice during war includes the requirement of a good faith willingness to parley in order to end it, if all the political aims of the war can be achieved without further killing.