But to add a comment about the OP's question. I've found that the prices don't go down on Norwegian if you are just a few months out and only go up as capacity builds.
I have noticed that if you are in say Dec and looking for June that the price can go down in January/Feb and stay low. So pricing starts a bit higher if it's a long way out.
And as you learned paying in NOK is the best deal. Euro second and dollars the worst.