Okay, it took a few days, but I have now accepted that the marathon will be hot. So I'm now going with A., B. and C. plans...
A. Keep to my planned sub-16:00/mile avg pace for all four races. I've been averaging 13:30-13:45 lately, but I'm MUCH slower in the heat. But I looked back at all my brutally hot summer long runs and still came in under a 15:00/mile average, so I think this is still possible. The benefit of having been running in cooler temps is that I don't have months of accumulated heat stress stacked up against me.
B. Just finish. At any pace that prevents massive heat stress. I need to put this in writing and say it to myself repeatedly, because I have a hard time accepting limitations on the spot. I know what "bad" feels like, so Plan B is to recognize it if it happens and act accordingly.
C. Bail. If it all goes very, very south and I just cannot handle the heat, bail and live to run another day and try to do so without regrets, knowing I did all I could.