1. The inward loco works back to Margam, so the loco arriving on 6M76 around 08:00 will return on 6V75 around 09:30.
2. I don't know the exact detail of the internal shunts between the reception sidings at Dee Marsh and Shotton steelworks itself, but I think a full train seems a bit heavy and long to be conveniently shunted with an 08 (which I assume they are still using there). I suspect they move half the wagons at a time, similar to this photo of an 08 with 13 wagons (trains usually run with up to 25 loads):
https://www.flickr.com/photos/195339731@N04/53615256790/
This 2006 view of the coil warehouse indicates you can get about half a train in there at once:
https://www.flickr.com/photos/tunnyspics/4605963293/
3. There is usually one train a day into Dee Marsh. The wagons arriving on 6M76 in the morning will normally work back on the next day's 6V75. The loco arriving on 6M76 will pick up wagons that are already waiting in the exchange sidings. The traffic is loaded coils into Shotton, the majority of which are loaded at Llanwern. Between Margam and Llanwern, the train may have other traffic on it (such as coils from Port Talbot for offload at Llanwern), so the wagons leaving Margam on a Shotton train are not necessarily all for Shotton! 6M76 will often drop some wagons off at Llanwern and pick up different ones that have been loaded there for Shotton. So a coil might be hot rolled at Port Talbot, sent to Llanwern by train for cold rolling, then sent to Shotton (there are different processing lines involved at each location; Shotton will be taking cold rolled coils from either Llanwern or Port Talbot that were originally hot-rolled at Port Talbot from imported slab).
4. At Shotton, sometimes the train returns to South Wales completely empty, but it's common for Shotton to reload a handful of wagons (i.e. all the wagons arrive from South Wales loaded with coils for processing, and a few are "backloaded" with finished coils that have been processed at Shotton). Tata says that the site "manufactures approximately 500,000 tonnes of metallic coated and pre-finished steel per year for building envelope, domestic and consumer applications" so I guess they would be despatching coils with special coatings by rail. Typical destinations for these would be Llanwern, Round Oak or Newport Docks; for example a loaded wagon for Round Oak would return with the empties to South Wales to be attached to one of the usual Round Oak services. It's an economic way of moving small volumes on existing services. There's a steep gradient for trains leaving Dee Marsh, which means they couldn't reload more than ~10 or so of the wagons.
5. With one train a day there's plenty of time for the wagons to be emptied and back in the reception sidings, I think they would normally be back there by late afternoon. Occasionally there might be two trains into Dee Marsh on the same day (6M76/6V75 in the morning and then 6M86 arriving in the evening and returning as 6V80). If this happens, the wagons that would normally form 6V75 the next day will be taken by the evening 6V80, leaving a shorter train of empties for the next morning's 6V75, depending how many wagons are on site. I don't know what hours the internal railway operates, but it's possible they only operate it for one shift during the day, which would be adequate for the usual amount of traffic. Main line locos will most likely pick up available traffic with the intention of a right time departure, rather than wait around for additional wagons to be emptied.
This picture and the caption (by Vincent Hardy again) give some idea of the operations -
https://www.flickr.com/photos/195339731@N04/53591642872/
6. Normal working would be 6M76 / 6V75 running up to six days per week. There might occasionally be a second train running on the 6M86 (arrive 18:47) / 6V80 (depart 20:39) paths, but not very often.
Tata Steel Shotton Introduction (brief clip of an 08 entering the warehouse with 10 coil wagons at 2:39) -
