Since you are on a special fare, and NW agents have to book it. I would think that you will have to fly on NW/klm metal when possible. Most special fares and discount certs are only valid on the issuing carriers planes.

And you are correct, you generally have to keep moving the same direction, so i do not think they would book you dtw-lax-msp-onward. I think you could choose either dtw-lax or dtw-msp and then on to NRT or Hawaii.