Neither park closed early on grad nights this year. The actual Grad Nite event was held in DCA from 10 PM - 2 AM which was after the park's regular closing time.
The grads all had park hopper tickets so most spent the day in DL. The mornings were fine, but we saw more grads beginning in the afternoon and evening. It was noticeably more crowded in Tomorrowland, around Indy, Big Thunder, Splash and any other thrill rides so if you are planning on going any of them, it is best to do so in the morning.
As far as fireworks, at this time we don't know what DL will do next May. Usually DL only does fireworks Fri, Sat and Sun evenings in the offseason and doesn't begin nightly fireworks until the start of the summer season which is Memorial Day weekend. This year because of the 60th anniv DL has had nightly fireworks since May 22, but there's no way of knowing if they will continue after the holidays or go back to their regular off season schedule in January.