Like I wrote earlier, check the unused numbers. You can just walk in and get a permit if you are going most days in September. In July and August, especially weekends, they draw numbers out of a hat and process people in the order drawn.
No, 40% are not held back for walk-ins. On the Main Mt whitney Trail, the permits are 100% reservable. The 40% situation is for all other trails for overnight use. But trust me... the no-shows and cancellations make up for that on Mt Whitney. Read the "Wilderness Permit Options" link in the Feature Topics box (above on the left).
In Sept/Oct, you will find water in all the usual places, except for the spring on the switchbacks. So last water is the tarn at Trail Camp.
