That's an ambitious travel schedule, to say the least. I suspect nobody's going to enjoy that 16 hour drive. I shudder at the prospects of leaving at midnight. But, hey, it's your trip not mine.
I've been to many of those places, and they are all beautiful. And yes, they certainly will be hot in the summer.
The number one recommendation I can make is to to keep your travel plans flexible. Be prepared to change your plans, stay an extra day, or leave a day early (an unlikely event, IMHO, but still . . . ).
And with the route you've plotted, you might consider adding the North Rim of the Grand Canyon. It is 2,000 feet higher in altitude, and very much different from the South Rim.