They will start taking down the decorations around the time you leave on the 7th, if it is like other years.
We have spent many NYEs at WDW and always spent it at Epcot. There is a lot of space there and yet it is still very crowded, so I cannot begin to imagine what it is like at MGM or MK, crowd wise! There are two fireworks shows that night, an earlier one for families who want to leave, and then an enormous one leading up to midnight. There are also special bands and activities all over world showcase all evening. Don't forget to get a glass of champagne in France before the midnight countdown!