<<So do some pilots request to stay in the "stack" at say 0530 ? Bet that gets complicated, aren't they all descending while they hold? >>
Aircraft are instructed to hold if there is a delay, so they simply enter the published holding patterns until they can commence approach for landing. Nothing at all complicated. They are not necessarily descending, but when they do it is carried out in accordance with standard procedures and separation is maintained at all times.