In the excellent book "Racing for the Bomb" it seemed like General Leslie Groves was confident of having several more nearly complete by the Hiroshima and Nagasaki strikes (sites he picked for strategic reasons) and a total of 7 additional for 1945 sounds right, I suspect the rest would be available after the invasion had commenced.
Aerial bursts over what remained of the Japanese surface fleet and large concentrations of troops in the first few days seems logical, possibly several key major airfields or aircraft production centers (taking the lesson of what to bomb from the last air campaign against Germany) while I'd expect we'd continue to do thousand plane firebomb raids against the cities given the logistical base was already there at Okinawa and quite a bit more of the European theater's bomber fleet would have transferred over to the Pacific theater by later in 1945. Given the firebomb raids killed more people both in individual attacks like Tokyo and Dresden, and cumulatively as well, I think the advantages of single plane delivery for the atomic bombs would have steered them to more mobile targets like ships and armies than easier cities and factories. I thought I'd read we were already running out of strategic bombing targets in Japan by OTL's war's end.
It would have killed a million more people which in a war in which at least 50 million died, probably doesn't change the realpolitik afterwards much at all. One man dies it's a tragedy, a million die it's a statistic (I forgot what wise man pointed that sad fact out.)
Nuking the beaches immediately prior to the invasion seems pretty crazy, waves of B-29's dropping incendiaries and napalm would be cheaper, faster, devastating, and allow much faster exploitation of the now mostly undefended beach (also handier against bunkers, landmines, cave systems, etc.)
A friend of my dad's was an 18 year old Marine fresh out of Pendleton waiting in Okinawa as part of the first assault wave. As novices as well as initial wave, they figured to be dead by the end of the first day at best.