You'd be very lucky to find the gap to have a loco hanging about on the main line while coupling/uncoupling wagons. Especially at Oxwellmains. If keeping clear of the main line you'd need a long headshunt. It might not have to be 2x the length of the train, but you wold need it to be long enough for Loco + longest number of reach wagons.Not necessarily. The reach wagons can be kept on an adjacent siding and just pulled back and shunted onto the end of the train. If the freight facility is quite close to the running lines the loco may even be able to stay on the main line while positioning the load.
Think simple. Let's not Gold Plate solutions.
The simple solution is battery-equipped locos. No need for idle (wasted) rolling stock at each and every siding then. The batteries don't even need to be very big (they could be charged when running under OLE), and provide the resiliance benefit that in the event of an OLE or Pantograph isolation/damage the train would not be left totally stranded.