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.
