For
#1 , there is no legal requirement to allow a passenger to disembark until you hit the three hour mark for a tarmac delay. However, there are several reasons Delta might do it even though they aren't legally required to:
1) The crew might have worried the passenger would cause trouble if they didn't
2) There has been a lot of bad press about passengers and airlines crashing and they may be more cautious.
3) The passenger may have relayed other information that made their request more pressing (for example a medical reason...)