In most cases DL will grant ORC = original routing credit (please use those exact words to avoid confusion) fairly easily, but you might get more miles from the flights actually flown. Surely OP would earn at least a few more miles on the ATL connection than the nonstop and possibly even more if the rebooking in C class results in 200% MQM.

C is normally FULL fare business class and can be used interchangeably with J for this. On routes where both C and J fares are offered, J tends to mean last seat availability while C is the fully flexible fare that's changeable without fees and fully refundable without any fees. Of course, one should look at the specific fare rules to verify this.