I haven't been to most of the places covered by the cruise excursions, but I think others here will be able to help at least to some degree.
To get an idea of the Nutshell pricing, see what you can find on NorwaysBest.com. That will be the maximum you will pay, since doing it yourself will be cheaper. I think the fares for the Voss-Gudvangen bus, Flamsbana and Naeroyfjord ferry are the same year-round, but I could be wrong. I'm sure the Bergen-Voss train ticket is fixed-price. The fare from Myrdal to Oslo is variable in the sense that you can get a better price by buying early--and those trains do sometimes sell out.
The full Norway-in-a-Nutshell route does make for a somewhat long day, but it you don't necessarily have to break the trip. Sunset will probably be about 9 PM at the time you're traveling, so you should have decent daylight. While there are things you can see at stops along the way, many require enough time that you'd probably need to spend the night somewhere. In my mind, the trip is very worthwhile even if you don't do any extra sightseeing at the transfer points. The transfers are very easy; I made the trip solo as a 70-year-old and had no difficulty moving my (wheeled but not small) suitcase from one mode of transportation to the next.
I think Flam is the most desirable place to spend the night if you decide to do that, but lodging demand exceeds supply there, so rates are high. I snagged a basic room (but with bath) at the hostel in Flam, so I spent two nights there in order to take a side trip to the Borgund stave church; that's a bit tricky because of the timing of the (I think) one bus round trip per day. There are commercial tours to the Stegastein viewpoint, which others have liked. There's also a small museum in Flam about the building of the Flamsbana railroad. And of course there's good hiking in the area.
A few folks here have stayed in Aurland rather than Flam.
Voss isn't located on a fjord, but there are active-sport opportunities there. It's close to Bergen, though, so not positioned well if you want to break up the long Nutshell trip.
I don't think there's much at Gudvangen except the Viking attraction and at least one lodging option.
Given that you are currently planning to be in Bergen and then return home from Oslo, I think the Nutshell trip is an excellent use of a day. It doesn't seem desirable to me to take a pricey cruise-ship excursion that hits some but not all the Nutshell experience (none seem to include the Naeroyfjord ferry), return to Bergen and then take the regular train to Oslo, which will take at least 6 hours. I'd either do self-directed sightseeing around Bergen or look for a cruise excursion that doesn't duplicate major parts of the Nutshell route. The eastern part of the rail trip (the part from Myrdal to Oslo) is more scenic than the western part.