Reporting back with success -- I was able to book 2 low-tier economy returns to ATL from CDG by playing the ExpertFlyer card with a Delta phone rep.

My trip over is in the winter, so getting flights from ATL to EDI was no problem; I reserved those from the website.

I phoned Delta to request assistance with the return portion, asking the rep to specifically check Air France for July 1. She replied by telling me there was indeed space ... in First Class (just like delta.com had said). I said I'd been monitoring AF availability on Expert Flyer, and that it indicated there ought to be some economy seats available. Hey, presto -- she found 'em after all, and I am booked.

Thank you all for helping me out!

Love from a grateful kettle