15 Day Norway "loop" in late August & early September - plan for alt routes?

There is no harm in having back up options in mind using entur.

For instance Day 4 the Oslo to Flam train- there is also a scheduled long distance bus from Oslo to Flam. So if the rail line is washed out/has a rock fall etc etc that may be an option.

Day 1- CPH to Oslo flight. If it's cancelled check on the train options- CPH to Gothenburg, change Gothenburg to Oslo.

Day 5- One of the Norways best boats docks overnight in Flam and one in Gudvangen. So if the Flam boat breaks down they normally put you on the competition- Lustrabbatane. If you have to get to Gudvangen early to do it in reverse I seem to remember a very early school bus (which takes the public) from Flam to Gudvangen. Research that just in case of need.
Flam to Borgund- there are two different public bus routes. One gives minimal time there, the other too much, but is a back up- see entur

The norled boats- the timetable tells you which 4 boats run the service, with ship's phone numbers. You can track the boats on various websites such as vessel finder. So tonight I can see that the 0730 tomorrow from Leikanger (the MS Tyrving) is safely alongside at Sogndal. If she breaks down tomorrow you can then call the ship and see if they can put you on the 1218 to Aurland (for bus to Bergen) or the afternoon 1630 to Bergen. You will also be able to see her leave Sogndal at 0650.
If the Flam to Leikanger breaks down then take the 1830 bus from Flam, change at Sogndal. Through vessel finder you will know hours beforehand if there is an issue, so can book the bus soonest. If need be call the ship for advice.

Likewise Day 9- Fjord line are good at updating any delays on their website, but you can also use vessel finder to track the ship on the way from Denmark.
If need be there are several direct double deck buses a day from Bergen to Stavanger, but Fjord line would more than likely provide land transport to Stavanger, Egersund or Kristiansand.

If push came to shove train via Bergen (overnight if needs be Oslo to Stavanger) or Norwegian Air Shuttle, SAS and Wideroe all run puddle hopper flights between Bergen and Stavanger.
Wideroe also fly from Stavanger to Sandefjord (Oslo Torp) for trains back to Kristiansand- in that type of emergency maybe/hopefully rent the car at Sandefjord instead.

That's what I would do, have a suite of back up plans in my back pocket which I am hugely unlikely to need, but can pull straight out if the worst happens. The alternatives may be sub-optimal but if they get you there, that is all that matters.

Those options are so helpful to have! Thank you!!

I recently returned from a solo trip doing Norway in a Nutshell as the core middle section of the itinerary.
Great you are spending time in Flam. Where are you staying?
I did EES in and out of Copenhagen. Line getting in (doing photo, fingerprints and passport scan) took about 20 minutes BUT I got off of my flight early on, and large line behind me as I was going through.
Leaving, very confusing as mass of people not sure where to be, which line, etc. Staff came by and gave some direction. Then, as it was taken a very long time, staff came through and pulled people to front based on boarding time of their flights. So all worked out, but use restrooms BEFORE getting into the lines! As others have states, this is fluid depending on what airport, if equipment working, time of day, etc.

And I have to mention, as you are a librarian - IF you have time the Royal Library tour was fantastic!

Thank you!!

We are staying at Lunde camping in Aurland, so I got some helpful tips on bus options between Aurland and Flåm in this post earlier:
https://community.ricksteves.com/travel-forum/norway/sognefjord-in-late-august-public-transit-options-between-flam-aurland-etc

I definitely have libraries bookmarked all over and since I have a supervisor who was recently in Oslo for an international librarians conference, I have gotten some recommendations from her as well.

It's very helpful to have those tips about the airport passport lines & processing, I know it can vary a lot by day and time (Chicago airports are definitely the same way) but I was hoping it wasn't something I should worry about upsetting our layover plans, as long as we plan for extra time in case lines are longer.

Emily - next trip include Helsinki and get to the Oodi Library - absolutely incredible! You can borrow guitars, etc and they have a sound recording studio.

And I stayed one night in Aurland, two nights in Flam. Loved both! There is Fjord Farm in Flam when you are there, and the Flamsbana Museum for your mom! It is really interesting.

Thanks for the extra tips! One of my friends says it's always essential to leave something for the next trip, so you don't feel too disappointed if you can't fit everything in.

I didn't book anything specific that first afternoon in Sognefjord. Since we will arrive in Flåm on a day without any cruises in town, I have a couple different options in mind depending on weather, including the Flåm Farm and local hikes. Or we can just take the bus to Aurland to check into our lodgings and poke around there, pick up groceries at the local market, etc.

We figured the train museum would be an easy option the first day or we could go with crowds present on other days between trips.

I wasn't sure about trying to fit in a visit to Otternes, (between Flåm and Aurland) since it's a bit longer hike and would have to factor in luggage the days we arrive and leave. Plus I wasn't sure about bus options on weekends.