Hal’s post last Friday led me back to the exhausting and exhaustive biography of Emperor Hirohito by Herbert Bix.
Bix had tremendous access to documents from the imperial household. He
notes the firebombing of Tokyo as well as the American desire to use
the atomic bomb as much to demonstrate its capabilities to the Soviet
Union as to force Japan to capitulate. What he does not do, however, is
say anything about an offer of unconditional surrender in January 1945.
Two quotes on this subject:

Thus he [Hirohito] firmly rejected [a February 14] recommendation that he act immediately and directly to end the war.

It was only after the Battle of Okinawa [in mid-June 1945]
had been fought and horribly lost, leaving huge sections of more than
sixty Japanese cities leveled by American incendiary air attacks, that
Hirohito indicated his desire for peace and started lookng for ways to
end the war.

Descriptions of events in the 1920s era of naval treaties, the 1930s
campaign in China, and even through 1941 show that war was not
preordained, but was the result of conscious decisions by the emperor.