We had the same situation on our last trip. It was about 9:30 at night, MK was closed. We asked at the Poly if they could call a cab. Instead, they got a wdw van and took us back to POR themselves. We were very impressed. Now, maybe they were just bored and wanted to get away for a bit, but those two guys were terrific. My question would be....how late do the resort buses run after the park closes?
Busses will run until the park is clear of all resort guests. Typically this can take 1.5-2 hours. However, because we could clear at any given moment, to avoid anyone getting stranded, the monorail will only drop off at the MK for one hour after the park closes. After that, they will not allow a drop off.
The fireworks from the Magic Kingdom can be viewed from the Contemporary, TTC, Polynesian, Grand Floridian, and the front cab of the Epcot monorail (if you time it right)