You're overflowing with good insight on this thread, Gareth! Let's see if I can add some of my own...
If first class is only going to be provided two-hourly between Swansea and Manchester, then it must either be on the Milford terminators or the Carmarthen ones. After a run of just over four hours from Manchester to Swansea, the first class unit would arrive roughly twenty minutes before the next service towards Manchester. I don't think it would couple to that service for two reasons. Firstly, twenty minutes could easily be lost during disruption, leading to lack of provision for first class passengers or the need to detach/attach the first class unit earlier (e.g. Cardiff?). Secondly, going straight back up would result in a nine-hour round trip, which doesn't suit a two-hourly service.
So working on the assumption of an extended layover at Swansea for the first class units, a ten-hour round trip would require five allocated diagrams for a two-hourly service, which matches nicely with the theory that the final seven 2024 units are the first class ones.
The next question is which services will have the first class, the Carmarthen or Milford services? Disregarding rush hour flows, one would expect more patronage on the Milford services: most passengers for west of Carmarthen will aim for these, whereas those who stay east of Carmarthen will be equally spread between both. As the minimum length of a train with detachable first class accommodation will be 4-car (2+2), I expect that the first class accommodation would be on the Milford services between Swansea and Manchester (but only 2 cars going to Milford), with the Carmarthen terminators being diagrammed for 2 or 3-car according to demand.
(Another thought: a fully homogenous long-distance fleet means that a spare unit could easily be attached to an existing service for a portion of the journey (e.g. Swansea - Cardiff) at peak hours, with no need for extra paths/drivers. I think there's something similar done at Mach now with a 158 doing one morning diagram before going on maintenance for the rest of the day.)