For the NiN part of the trip, there have been reports of sellouts on the Flamsbana (Flam to Myrdal) and on the regular trains between Myrdal and Oslo. I think just recently someone reported a full Gudvangen-Flam ferry, but I may be wrong about that.
I assume you are planning to complete the NiN route in one day. It's difficult to find conveniently-located lodging to permit splitting of the journey. Flam is most convenient, but it sells out months in advance--though scoring a cancellation is possible if you are lucky.
I didn't take many intercity buses in Norway so am not in a position to comment on their general reliability. I did use a bus to go to the Borgund Stave Church while I was staying in Flam. The bus departed Flam 20 minutes late, which wasn't horrible, but it cut my time at the church from 60 minutes to 40 minutes, so I didn't have time to see the stave church museum. That bus runs only once a day. When you get out in rural areas, some buses are very infrequent. That obviously varies from route to route, so you need to dig into the details as you develop your itinerary.
Quite a few people on the forum have driven in Norway and reported on their experiences. It seems quite doable if (unlike me) you aren't spooked at the idea of having to back your car down a mountain highway so a bus can get around you. The easiest way to find earlier reports on driving in Norway might be just to skim back through the Norway forum, looking for pertinent thread titles.
Nothing is cheap in Norway, and that definitely includes car rentals. But it also includes bus tickets, so...
