Per labor agreements, KLM can not "take over" an NW flight without NW replacing KLM elsewhere. Perhaps the fifth DTW-AMS flight will allow KL to operate from one of NW's hubs, but I think it's doubtful.

NW/KL do not split cargo revenues. KL was contracted to carry cargo for (but not limited to) JNY and Kuhlman to several MSP-based distributers. I believe these contracts have since ended. That played a big role in KL's desire to serve MSP-AMS. When their seasonal service ended, NW added a third frequency... probably in response to ATL-AMS & its labor contracts.

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Too bad NW's parking a B744 at JFK for 23-hours and not flying it to AMS, like they use to (on days it wasn't undergoing mx).