Death Valley is something you only want to do when the weather is cool. Between now and November, I'd drive around it.
While I don't mean to sound argumentative ... if you fly into Las Vegas with Whitney as your destination, you don't really have choices which will avoid warm temps. The temps in Vegas are generally within 5-10 degrees of DV since the temps are mostly a function of elevation, with a 3-5 degree change per 1000' of elevation. For example - as I type this, it's 85F in Lone Pine, and 94F in DV. LP is 3.8K', and DV at the Visitor Center is a bit below 0K'. That's a common differential. And, the temps in Las Vegas are 95F, so DV is a bit cooler than Lost Wages.
And while I wouldn't want to drive thru DV without air conditioning, I wouldn't want to drive thru Lone Pine without it either. But, in this day and age it would be tough to find a rental car without A/C.
And don't forgot - not all of DV is blisteringly hot, even in August. Driving over Townes Pass is 5K', and it's marked cooler there. Or at the Charcoal Kilns (7K'), or Mahagony Flats (8K'). It's quite "safe" to climb Telescope in summer - just bring 3L or so of water. After all, with a start of 8K and a summit of 11K, it's not unlike going from the Portal to about Trailside Meadow, something scores of people do every day in summer.
