Norwegian has their own reservation system, which no other airline has access to. An airline can only book you onto another airline when it has access to the reservation system.

Which basically means that when there's IRROPS at SAS, SAS can book you onto all IATA members like LH, UA, KL, BA, CX, AY, TG, and hundreds of other airlines. But not airlines like DY and FR, which have their own systems.

In the same way, if you have a ticket with Norwegian and they have IRROPS, they can't book you onto any other airline at all, only their own flights.

So, if you don't want to book another ticket with SAS because they can't rebook you onto DY, you basically cannot book a ticket with any airline at all, because there is no airline in the world that can rebook you onto all other airlines.

Travellink, on the other hand, is a low-cost booking site, meaning that they don't even list a telephone number that people can call when they have problems. If you book with such a low-cost site, you're taking a risk in order to save some money. If you don't want to take that risk, always book directly with the airline.