Yes, the southern Sierra / Piute Mountains are really arid and dry, especially that late in the year. When I did the PCT, I went through there in May and I thought it was one of the driest spots on the entire hike. So I definitely would think about bailing after the High Sierra. Whitney is only a couple of days north of Kennedy Meadows, so it wouldn't be bad to summit Whitney and then just head down to Whitney Portal to hitch down into Owens Valley. Or go to Kennedy Meadows, or to Hwy 178 at Walker Pass and hitch-hike down.
Six weeks south from Donner will give you lots of time to go leisurely through the Yosemite hinterlands and the High Sierra. Nothing wrong with that. If you are adventurous, you could add in parts of Roper's Sierra High Route, to get you off trail and keep you up above timberline, instead of some of the down-in-the-valley JMT sections. Would also get you away from some of the crowds. There are some good, easy peak climbs along the way too. And lakes to nap on warm granite by. Etc. I'm happy to share some ideas if ya want 'em.
Sounds like a great trip!