JB, you can still be loyal to Delta indirectly by using one of their SKYMILES partners, like Alitalia!
I lived in ATL for years and flew 4-5 times a year internationally out of ATL. Having booked flights often from ATL, I can tell you that Paris is an expensive site to fly into (I blame Air France and CDG!) Also, the week of December 20-27 is one of the most expensive weeks to fly BACK to the States. We fly back into ATL annually at XMAS each year... and use "miles" for those trips - avoiding the higher costs.
If you go into Gatwick you may get a better rate, but you'll have to add in the cost of flying or train (Chunnel) to Paris. If you could move your December flight forward even a week you'll save more money!
That said, on www.vayama.com I found flights on NW (SKYMILES partner - and future part of Delta) for $1266 all in doing your "original" itinerary (MXP return). If you insert Gatwick, it's $1205 all in.
FYI, You can fly out of Charleston on US Airways through Charlotte, and back through Philadelphia for $1241. You can also fly out of Myrtle Beach for $1246! Not sure if that is more convenient for you! In my previous life I flew weekly so I have a ton of search engines I've used...
Hope those suggestions help...
Ciao,
Ron