Living in NYC, i always buy DL tickets to fly AF to CDG, because of the stupid 25% miles rule on cheapt fares AF tickets ! So, with a DL issued ticket, you be be able to get your seat assigned by calling a DL rep on their hotline.
Indeed, i just had my seat booked both ways for my next JFK/CDG on DL8603/AF009 and DL8532/AF10. Those seats were confirmed by DL rep, but not by AF rep when i called them (they told they could not access DL infos...). So, there is still a doubt but it should be fine.
You should call DL again, you might just find a more helpful rep. if you have elite status on FB, you could also try to ask talking to elite skymiles rep, they are sometimes more helpful... good luck