I'm not sure which response to say is best.... some of the information is confusing.
As long as the reservation that you got in the lottery, or on-line after the lottery, is for a Mt Whitney Trail Overnight Permit, then the ONLY thing that matters is that you pick up the actual permit at the Visitor Center just S of Lone Pine (the day of, or up to two days before the start date), and then START on the trail at Whitney Portal on the specific start date.
Even if the reservation states the number of nights, nobody cares about that. You can change that when you pick up the actual permit. (You can even decide to change your exit plans while out on the trail.) The number of nights spent on the trail is informational only, and has no effect on the quotas.
You can stay overnight on the trail as many nights as you need or want (there is only a limit of 14 nights in the same place). When you pick up the permit, they ask you for your planned exit date (and write it on the permit, I think). It does not matter what the reservation has for number of nights.
Once you are on the trail, nobody will mind if you come out early or late, as long as family or friends don't call in a panic and report you missing if you come out late. People have reported wishing to stay an extra night beyond what their permit states, but getting out because of what was on the permit. They can stay the extra night without any problem, except for the issue of family/friends reporting you missing.
Again, the permit reservation is ONLY for a permit to start on that specific date.
