For some reason the select seats function hasn't worked on the Air France US site for the past few months (in fact it doesn't even show up as an option). As you've probably seen, you can chose seats on KLM.com, but as you mention, the KLM seatmap often has issues recognizing DL status. You should be able to call the DL PM line and have them assign you Economy Comfort seats on KL flights and Seat+ on AF if they are available. However, I've also noticed lately that AF blocks a lot of the cabin for pre-assigned seats until last minute.