Questions about post-tour travel in the Sogneford

I didn't travel by car in Norway, so I can't answer any car-related questions. There may be a bus between Balestrand and Sogndal, but it might run only once or twice a day. That can happen in rural Norway.

The tour includes a 2-hour ferry trip on the Sognefjord. I assume that's the highly scenic trip between Flam and Gudvangen (down the narrow arm of the Naeroyfjord, an offshoot of the Sognefjord), but the itinerary doesn't specify that.

The Norway-in-a-Nutshell itinerary includes four additional segments, besides that ferry trip. Three of those segments are very scenic. By taking the express boat rather than following the classic NiN route (and substituting an extended train trip or bus ride for the Naeroyfjord trip you will already have taken) and later taking the train from Myrdal (I assume) to Oslo, you will apparently miss the scenic bus ride from Voss down to Gudvangen. When you're ready to go on to Oslo, you may be able to take the other two scenic legs of the trip, the Flamsbana train from Flam up to Myrdal and the main-line train from Myrdal to Oslo. That would mean you only miss the 1-hour bus ride, and having your own wheels for 1-1/2 or 2 days surely will make up for that.

The train trip from Bergen to Voss isn't spectacular, and the NiN route necessarily skips the small Voss-to-Myrdal segment of the rail line as it routes you down to the fjord and on the ferry. I haven't seen the Voss-Myrdal scenery myself, so I can't tell you how much you're missing there, but I'm sure the book doesn't describe that part of the rail line as spectacular. I believe you're thinking of the Flamsbana from Myrdal down to Flam, and you should be able to fit that in.

I'm sure the express boat isn't non-scenic, it's just not as scenic as the ferry through the Naeroyfjord, but I think the tour gives you that. And with a car you'll be able to cover a lot of ground in a very beautiful area. I would just caution you to be very careful about your transportation to pick up the car and the transportation from the drop-off point to whichever ferry you're going to use to get to the south side of the Sognefjord, which is where you'll pick up the train to Oslo. Buses and ferries may be infrequent enough to cause you problems.

Become realistic. We speak about 1 full day and 2 nights.
Sorry but your "plan" makes no sense to me.

Neither Sognefjord nor Balestrand have advantages compared to Hardangerfjord and places there, e. g. Norheimsund, Utne. The way to Balestrand for one night only is not worth it compared to other places imo.

The much better idea is to rent a car in Bergen. Overnight in Utne with fjord view via Norheimsund. Next day do parts of Hardanger route (ferry Kinsarvik, Hardanger bridge, Skjervsfossen) with following Flam, Flambana and Stegastein by car and head back to Voss to return car. Stay in Voss and travel next day by train to Oslo. Much more experience by less travel stress.

If you want to do this Balestrand night.
Take train Bergen - Voss
rent a car there.
Drive to Balestrand via 13 and ferry Vangsnes - take care of last ferry (stave church Hopperstad on the way)
Night in Balestrand. Drive to Kaupanger and take pre-booked ferry to Gudvangen. Drive to Flam and do Flambana there.
Back to Voss, return car and take train to Oslo.

Good planning and journey.

I was on the Scandinavia tour last August, and did the NiN to get back to Oslo after the tour. As mentioned above, the tour includes a 2-hour ferry trip on the Sognefjord.

My main thought was that if the weather wasn't good on the tour day I would have another chance to enjoy the fjords. As it happened, weather was great on the tour day and rainy/overcast on the NiN day. I still feel it was the best choice, the scenery, especially on the bus part and the Flåmsbana part were spectacular. It was a long day, but worked for me as I was traveling solo.