The problem with starting B-29 operations with atomic weapons from the Marianas in late 1944 is that the USA does not have Iwo Jima. Having Iwo both eliminates some Japanese early warning as well as providing an emergency landing strip. OTL the first emergency landing there was even before island was completely secured. Of course having Okinawa is even better. Also in late 1944 Japanese home defense from a fighter standpoint is not as crushed as it would be the next August. Given the problems the B-29 had, and silverplates were not immune from this, a mission from the Marianas in late 1944 has a lot of risks even if the Japanese don't shoot them down.
Assuming that the US does drop a bomb on Japan. I expect that the Japanese would look at where they are - US forces and forces in SEA are atill away from the core and also seized areas. I expect that once the Japanese twig to what a B-29 can do, you'll have Kamikazes crashing in to B-29s every time they appear. Also in late fall 1944 Japanese cities have barely been touched by strategic bombing, the atomic bomb was icing on a large cake of destruction.
I am not one who says that Russia piling on in August 1944 (between Hirsohima and Nagasaki) was the deciding factor. For sure as loing as the USSR is fighting Germany in fall, 1944 they are NOT going to be attacking Japan. The agreement they had with the USA was coming in 90 days after Germany surrendered, and frankly while still trying to crush Germany the Soviets can't do anything to help. In fact if they do join the war in the Pacific, then the lend-lease that was shipped on Soviet freighters fro the US across the Pacific goes away.
Sloreck, you made some points!
I believe the US is going to study the Silverplate missions against Germany and adjust their planning for nuking Japan.
The early raids against Japan in late 1944 were not that effective OTL. There will be greater pressure on the 20th Air Force to get results in TTL. So maybe we get a nuclear attack against Japan beginning in the Spring of 1945?
Could the Iwo Jima Operation be moved up by delaying the Philippines campaign? If Roosevelt and Marshall know that the bomb will be ready in the summer of 1944 would the Marianas and the seizure of Iwo Jima take precedence over MacArthur making his great return? Iwo Jima is captured in November 1944 and the P-51s brought up there to provide fighter support to the B-29s. In February 1945 the US Navy's Essex class carriers begin operations off the Japanese coast. Silverplate B-29s escorted by Mustangs based out of Iwo Jima carry out their mission.