I see the permit as either ALLOWING you to be on the mountain for one day, or for multiple days. A permit that allows for multiple days does not prohibit you from being on the trail for fewer than multiple days.
Imagine you started hiking the first day permitted on your permit and you got back NEAR the trailhead 5 minutes before midnight. A ranger happens to stop you and sees you have an overnight permit. I highly doubt he would make you pitch a tent and sleep for 10 minutes before continuing to exit the trailhead the "next day".
More importantly, I think you just need to decide whether you will dayhike it or not dayhike it. The decision alters your start time, your pace, and where you set up camp. It doesn't make sense to carry your full gear 'just in case' you get too tired and decide to sleep a night.
To me a 2-day hike is just as strenuous as a dayhike of Whitney since you have to carry so much weight for so far/high. If you carry your gear AND dayhike, then it will more strenuous than either option alone, which reduces your likelihood of a successful and or enjoyable hike.
