Questions about booking transatlantic & in-Norway flights

If I were doing this, I would just plan to stay a day or two in Oslo before the onward flight to the Lofotens.

I book my own flights, but I reduce the risk of delays by staying at least a night before an onward flight on a separate ticket.

Flights within Norway normally maintain their schedules. ( except in 2010 when a volcanoe in Iceland spewed so much ash into the atmosphere that flights across Northern Europe were disrupted and rerouted). So, the question is what is the on-time record for the airline you’re traveling with from the US to Oslo and will you be traveling when weather delays are unlikely.

Travel insurance would cover the cost of the unusable airline tickets if a trip delay caused you to miss your onward flight. The only problem is the substitute tickets bought on your dime the day of the flight are going to cost an arm and a leg at the last minute.
And there may not be any seats remaining on that flight. In any case, almost all the flights from Oslo to Svolvaer (SVJ) Airport and Leknes’ Airport in the Lofotens will stopover in Bodo and maybe Trondheim and that is going to add several hours in the air after your transatlantic flight. Not something most folks would want to pack into a single day.
The safest choice is to overnight in Oslo after arriving there. There are several hotels right by Oslo’s Sentral ( Oslo S) Train station which is near the waterfront and many of Oslo’s attractions. The direct express train called “FlytoGet” from the airport directly to Oslo S Station takes just 25 minutes.

A travel agent can issue a ticket which otherwise you may not be able to book yourself IF the two airlines have an Interline agreement. In the past I was able to book such a ticket [Air New Zealand to British Airways]. I found the fare on Expedia - it was labeled "multiple airlines," But a travel agent can't simply combine two flights onto a single ticket if the airlines don't have an agreement between them.

Did you check if there are any code shares you can use? I once wanted to use a direct flight out to Zurich on Swiss Air, and a direct return flight from Frankfurt on Lufthansa. I was able to book both those flights together under United. However, I don't know your airlines.
Don't use a third party agent to book tickets, ( Expedia, etc.) always book directly with the airlines. If you have a canceled or delayed flight, it's a big headache working with a third party broker.
In addition to flight cancelations and delays, there is an increasing risk of airlines just changing your flight times after you book. For caution, I would fly the last leg the next day.
Flight Aware can help you research a flight's on time dependability.
Have a great trip and good luck!

I do not know how a travel agent could help you in this topic; maybe more for your own feeling.

Transatlantic flights eastbound are more often in or even before time compared to westbound (against main jetstream direction).

Flights within Norway are normally operated by SAS, Norwegian or Widerøe. Only SAS could have code shares but that would not help at delays. SAS declared the days to leave Star Alliance and to join SkyTeam.

Tip #1: Fly in directly to Lofoten islands (e. g. Svolvær), not Bodø with the need to take a ferry for hours. Also possible but some way to drive is airport EVE (Narvik / Harstad). I used this one and it was good.

Tip #2: Besides Oslo also Bergen is a well connected airport within Europe - in case you decide not to fly in via a Europen hub. Nice to stay in this region for some days.

Tip #3: With Flex tickets in one direction you can take a next flight

Tip #4: The Norwegian airport pages by Avinor gives you really good overviews of their flight network and other topics, e. g. transfer at Oslo.

Tip #5: Do not underestimate the eastbound jetlag and the Norwegian fresh air hammer - most tourists get really tired from the amount of fresh air in the first days - at the coast but also in the back mainland of Norway.

The best way is to not book a connecting flight with different airlines and instead book the whole trip on a single booking. That saves you the headache of trying to self connect. No need to use a travel agent to do that, you can easily do it yourself online.

The direct express train called “FlytoGet” from the airport directly
to Oslo S Station takes just 25 minutes.

The airport express train is called Flytoget, while I've seen people do it a couple of times on this forum there is no need to capitalize random letters in the name, it just looks strange. I've never seen people write LufthaNsa, EurOstar or IcelanDair.

Only SAS could have code shares but that would not help at delays.

That would help with delays as they would rebook you if you miss a connecting flight.