By the way, here's what we decided to do: RENT a car!
NorwaysBest confirmed that they don't have an excursion that does both the Nærøyfjord and Borgund Stave Church. They also don't offer the ability to build a custom tour for that. Also, they don't have a bus to Borgund, or a taxi, or a private transfer tours that would do that. So the phone call nailed down what they don't do. Which, frankly, was VERY helpful in making plans. What they did suggest was renting my own car. And as it turns out ...
Perfect! Fjord cruise from Flåm to Gudvangen, taxi from Gudvagen to Flåm, rent a car in Flåm, drive to the stave church and return in plenty of time (fingers crossed) to make the 1800 departure.
Why take the taxi back to Flåm and not the NorwaysBest shuttle bus? Of course, the return shuttle bus is already fully booked. 🙂
Car rental is right there near the dock: FlåmCarRental
Taxi was also easy to book: FlåmTravel
Hope this helps!