I wo was necessary not only for damaged B-29s, but hundreds of P-51s could run partial escort services into and out of Japan for the B-29s.
I'd say that Iwo was completely necessary based upon the situation at the moment. Okinawa, on the other hand, was far more costly with allied deaths being nearly twice what they were at Iwo, and by the time it was captured, the first a-bomb detonation was only three weeks away.
Top War Dept. and governmental officials knew that the a-bomb test was imminent when Okinawa was invaded, yet they still made the decision to proceed. Nimitz and MacArthur both knew the casualties would be much higher than any previous battle, and so did the folks in DC.
I'm not going to dispute the facts you present.
BTW: My Dad was on his way to Okinawa when the first bomb was dropped. It was still considered a war zone, so when the boat arrived, he got 2 weeks combat pay staying aboard the transport. That was okay by him; he and a buddy owned the crap table (actually just a rug) and were getting a piece of all the action.
1) I don't think anyone really knew how the Japs were going to react to The Bomb. Indeed, it took the Emperor himself throwing in the towel and the generals reacted by attempting a failed coup.
2) All of the islands from Guadacanal on were learning experiences. We were learning how to best take the Japanese apart in a land invasion.
3) Okinawa was going to be a strategic jumping-off point for hitting the beaches on the mainland. Dad heard he was going to someplace codenamed "Landau," and he was going to train at Okinawa in a mock landing before being shipped off for the invasion.
To the bigger question:
Was all this unnecessary? Well, yes. You can say that Normandy was useless if you figure that in a year and a half, we could have nuked Hitler.
Dragoon could have been canceled as well. The Anzio landings? Piffle. Sicily? Useless! What the hell were we doing in North Africa?
Rather than second guess these situations, the more intriguing question for me is Why Iwo Jima? Why Okinawa? Why was MacArthur treated as a God? What were the motivations of the players involved? What did they know? What didn't they know?
