You are within the 90 day window for both legs of your itinerary, so seats can be assigned. This is a perennial problem for NW passengers flying on KL metal.
FT posters have expressed mixed results with
calling NW, KL or AF directly, emailing KL ([email protected]), OLCI, etc.
For non-Skyteam Elite flyers, speaking with a NW agent by phone to request seat assignments currently incurs a $15
fee.
If that $15 fee does not bother you, I would call and request your seats. NW agents usually have access to the KL system for seat assignments. OLCI at 23:59.59 can always be a crap shoot for getting TATL seats assigned.