Excuse my local ignorance, but are you saying that the current Manchester - Cardiff - Swansea - Carmarthen (and beyond) service would bypass the current Swansea station?
As for the current Cardiff - Birmingham service, if stock were available, I'd extend the existing Nottingham - Birmingham service to Cheltenham, then on to Cardiff via local stations, to replace the ATW service. This would mean double the Birmingham - Cardiff service, and mean Cheltenham gets four trains an hour to Birmingham. So, one train would go Nottingham - Birmigham - Cheltenham - Cardiff (fast), returning slow to Cheltenham and on to Nottingham (and vice versa). Would probably only need one more unit (taking over the existing ATW DMU too), if a path could be found on the Selley Oak line, and save conflicting movements at Cheltenham, where this service curently reverses.