Breaks are determined by the A Captain. When the flight departs and how much over 12 it flies are looked at. Most of my flights with a double crew are just over 12 hours. We normally take only one breaks. If we do take two breaks normally we do a short and a long break. The crew flying however always does both landings and takeoffs. The Captain who signs for the aircraft is in operational charge of the flight and I believe has to be in the Captains seat for TO and Landing. As mentioned normally the A crew flies over and the B crew back. On the really long flights like the 17:55 leg from Mumbai I am sure they do it differently. In the end however the A captain decides.
On the 8 to 12 hour flights its still up to the Captain. It used to be the Captain always took the middle break because it was quieter for rest. Over the years it has evolved and most but not all Captains have the pilot flying take the middle break. The relief pilot gets the first break and the pilot not flying the last break. On some flights in the 11 hour range crews will break the breaks up into a long and a short break. 1 hour break and a 2:30 break as a example. I don't like this but it is done.
One thing that does sound different at NWA from Delta. Since your have flights with 1 Captain and 3 FO's it sounds like FO's can occupy the Captains seat for TO and Landing. If you split the TO and Landing between crews. At Delta the FO's even though type rated can't occupy the left seat. In another weird thing the CA can never occupy the FO seat unless he has had dual seat training which is not normal unless you are a check airman.