I would purchase the NIAN deal b/c it's easier that way ... if you're not going at peak time, just buy it when you arrive. In my case, I ended up there during a strike of the ferry so they just left us on the bus for a longer portion -- I wasn't pleased (and had to explain to an Italian couple what was happening - I don't speak italian!) but I had prepaid and they weren't offering refunds.

Yes minipris is the sale -- I used the option to pick up the ticket at "my seat" and had no problems. If you're going direct to the airport, I'd just fly and not bother at all with the train, especially the overnight. During the NIAN you'll have the chance to see the scenery. In both Bergen and Oslo, the airport is a ride from the city. I used the airport bus in Bergen (from the airport) - there are a number of pickups all over the city; in Oslo it's the train.

It's beautiful country - enjoy your trip (but be prepared for the sticker shock, its a very expensive country to visit)