Was it necessary to use the atomic bombs to force a quick surrender or were there alternatives?

Answer 1

Yes it was necessary and yes there were alternatives, all of which the US was preparing to use to make Japan surrender.

The US was already conducting several 1000 plane firebombing raids every night (each of which killed as many Japanese as each atomic bomb), 23 atomic bombs were planned for production and use on Japan before the end of 1945 with more in 1946 and later if needed, 2 "D-day" invasions (one in october or november 1945 the other in spring of 1946) each larger than the Normandy D-day invasion of 1944, and stockpiling of chemical weapons (e.g. nerve gas, mustard gas) for use during the invasions if the Japanese fought as they had on several of the pacific islands, etc.

Also the USSR was attempting "backdoor" diplomacy on Japanese surrender terms, but these negotiations were proceeding slowly both due to Japanese demands for conditions (which the Potsdam declaration would not permit) and Stalin's foot dragging because he wanted as much Japanese land as he could grab militarily.

Had the Japanese not surrendered when they did US, Soviet, and Japanese troops would have soon found themselves fighting on the world's first atomic battlefield with about 2 atomic bombs exploding somewhere in Japan every week and the issues of radioactive fallout as well as poisonous chemical weapons gases. And if the atomic bombs not been used at all, chemical weapons would have certainly been used.