This isn't Summer, it's late Spring. Probably one of the better Windows to make this attempt. After most of the snow on the route has melted and prior to the warmest months of the year.
A month earlier and he's running through more snow in the Rockies and traditionally cold and rainy weather from Kansas east. A month later and it's brutally hot on the front end and muggy on the back end.
If I'm attempting this run, I'm looking at this as one of two ideal windows with the 2nd window Sept-Oct
I'd pointed that out:
I have a theory too wrote:
Choosing late spring/summer is the cover to validate the night running.
It's not a great time to avoid storms, the strong mixture of cold and heat associated with late spring generates some wild stuff. The heat of mid summer has it's own charms as far as storms are concerned. Ideally, run in the fall, as you mentioned Sept-Oct and allow daytime running for much better visibility for seeing and runner safety and much more visibility for verification of the effort.
Side question though, if he has this prowess for long distance running, why not run events like Western States, Comrades, or similar high profile very long endurance events? If he can afford to hop a plane to run London, then fly back to the states for that other event, clearly he has some budget available. Putting your name up there in high profile high participation events would give pretty good credibility and help far better with whatever causes he says he's supporting.