But first of all, are we entitled to $600/person in EU comp, or more?
Yes, EUR 600 per passenger!
AFAIK Norwegian doesn't interline with anyone, so they either rebook you on something acceptable from their own flights or give you a full refund.
This is not the problem of the passenger. Norwegian can certainly book a flight on expedia.com or directly on the airlines website with a corporate credit card.
Anyhow, the passenger may book the replacement flight by himself and claim that ticket costs back later, because Norwegian is violating EC261/2004 article 8 free rebooking onto the next available flight.
If it is Economy, Norwegian has to refund another 75% of the fare.