Just returned from the evening events and here are some pics.
The sing-along is a pretty basic stage set-up, with two large screens on either side and a movie screen behind the stage. Minimal scenery on the sides. The show mainly stars the 'Arrendelle historians' who look pretty similar to a couple of folks who hosted a Welcome Home Wednesday a couple years ago. They basically do plot exposition with a few puns to lead to the next song which is just shown from the movie - characters are not singing live until a Let It Go reprise at the end, but even that appeared to be totally lip-synched. It's fun - the kids love it, most of the parents were singing pretty loud as well.
From what we were told by the Premium door CM, all the shows today were totally full (can't really say sold out since they are free). The door CM at the dessert party said that the Premium tickets are selling VERY fast and that many dates are already sold out. Not sure of the accuracy of that...
The dessert party was set-up in Playhouse Disney due to the rain and I would encourage you to go on an evening when there is rain around, because this was MUCH better than being outside. Tables, multiple food and drink stations, Frozen dance music, and air conditioning.
At 9:30, the guests who were still at the dessert party were taken by CM to the fireworks area. The fireworks show starts with Anna, then some fireworks, then Kristoff joins, then some fireworks, then Elsa, and some more fireworks. Obviously, the weather tonight did not cooperate at all, with a low ceiling, little wind, and high humidity. But, through the smoke, it looked to be a decent show.