The fare on delta.com with the three segment return appears to be quite a bit a higher when I price it as the return flights are Q fares (outbound is a T fare) -- it comes out $1914 vs. $1344 for the one-stop return through CDG (T fares both ways). There's no real advantage to booking on delta.com, so I would pick the cheaper option on Orbitz (if it's able to book it).

I was able to bring up the same flights on ITA Matrix that you see on Orbitz, but when I use Matrix Tools to try to bring it up on delta.com, it returns an error. So I'm not entirely convinced there is availability in T on the Delta coded flights for the return. If I go to airfrance.com it returns the $1344 price when the flights are all AF-coded. I guess I'd see if Orbitz can actually book the tickets as DL T fares. If it bombs out, I'd just go over to airfrance.com and book them as AF coded flights. The mileage earning looks to be roughly the same (AF N and L fares) with AF option and the MQM earning will be the same 100%.