Definitely not a positioning move. Machynlleth crew have to be taxied up as only they and Pwllheli work ETRMS routes.
Indeed. I'm sure if it was just a positioning move they'd shuffle things round and have an extra set overnight in machy instead. You don't run a train in service even though it requires a lengthy taxi for train crew without a good reason.
That rather depends on how one defines positioning move, doesn't it? The fact is that the position the set in question is in overnight - at Shrewsbury - is not the position it needs to be in to work the 08.30 from Aberystwyth, which is surely the key reason the 06.25 exists, though as a spin-off it has its uses to passengers at certain stages of its journey west, such as Welshpool-Newtown if people have an early start at work, and as a commuter/college service on the last leg into Aberystwyth. It certainly does not exist to meet some overwhelming demand for rail travel west out of Shrewsbury at that time of the day.
There are plenty of examples of similar workings all over the country, especially early in the morning, where trains are officially in passenger service but travel a fair few miles before they have that many people on board, simply because of the position of overnight stabling points. Start the 06.25 at Machynlleth at 07.45 instead and you would probably need to find another set to cover what the 158 is doing the previous evening before reaching Shrewsbury, which is probably easier said than done with a fleet that is stretched thin to begin with.
What is the crew being taxied up from Mach meant to prove? ATW chose not to train drivers at Shrewsbury on ERTMS working, so they have no option.
Other TOCs use taxis to move crews similar distances. FGW stables three HSTs at Worcester overnight and another at Hereford, but doesn't have a drivers' depot at Worcester, so taxis to and from Bristol or Oxford are the order of the day.
And I can assure you that the 04.50 Hereford to Paddington is not run due to overwhelming demand from Herefordshire at that time of day, indeed the loadings on that service and the 05.11 from Worcester until you get to the vicinity of Moreton-in-Marsh and stations to its east are usually distinctly underwhelming, which will also be the case on the 06.25 as it pulls out of Shrewsbury for years to come.