I had an itinerary with no seat assigned, and I was able to get my preferred aisle seat assignment on my KLM AMS-LHR leg when I flew SEA-AMS-LHR a few months ago. The trick was to contact Delta - I actually was even able to use the web chat for this, but the elite line should be able to do this too - and ask for the AF PNR locator code. Your reservation should have a Delta entry (which is the six character code you know) and a corresponding KLM or AF entry that matches their flights (which is a six character code you don't know but need to find out.)

Once I knew the KLM code, it was easy to just log in to klm.com and choose a seat as if I'd bought the ticket directly from them. I presume it's the same story for Air France's web site.

The part of your situation I don't know is whether you need a KLM or AF PNR code, and whether you should use the KLM or AF web site. My guess is that you want to bypass the KL codeshare entirely and get the AF locator code and use airfrance.com to make your seat selections.

Hope this works for you - it did for me on a slightly less complex itinerary at least