In a perfect world, you could drop everyone off with all the luggage at 6:45, return the car at 7:00 and be at the gate by 7:30. But the reality is that you may encounter a galloping hoard of the unwashed masses all trying to do the same thing at the same time, and all it takes a couple of folks who have "issues" to throw a monkey wrench in the best laid plans.
But IMHO, returning the car to a live-person won't protect you from a false scratch+dent claim made later. The check-in person usually doesn't do more than check the gas guage, odometer, and issue a receipt.
IMHO, using a drop-box is okay, as long as you filled out a scratch+dent sheet at pick-up, prior to leaving the lot, and kept a copy for yourself.