It is important that your ticket be a NW flight number even though you are on KL;
1) It is difficult/(impossible?) for the NW agent to get to the KL "elite" seats
2) You can not see the seat charts on the NW web-site if it is a KL flight number. In fact you can not even see the seat chart at ITN if it is a KL flight number but you can see the seat chart if you are asking about the NW code-share flight number.
3) I have never had a problem with elite seating transatlantic on KL flights as long as I was using a NW flight number in fact based on my profile my next two flights have put me into 10a & 12a (my seats of preference) automatically more than 2 months early.
4) If the flight was not inexpensive for me (read cheap) magic111 would not be going.