Make sure you leave plenty of time to get through the lines at the airport as the queues are always massive.
In Nusa Dua you are out of the way a little and there isn't that much to do locally in regards to food. Yes there are places such as Bali Collection but there are not tons like in Seminyak.
I'm going to Bali soon and have 4 days in Nusa Dua to just sit at the pool and read. I'm then going to Seminyak so we can eat lots!
Sorry, not sure about airport lounges. We are in the Garuda J lounge which is quite nice. The location is great overlooking the airport but the lounge is not 5 star.
Have fun.