The reality is that nobody in Wales does not speak English
That's not true, there are many people in Wales who are not fluent in English and there will be some (very young kids and elderly people, particularly if things like dementia are a factor) who have little English ability.
And, the 175s were generally more comfortable than the 197s, even if they were too short.
Did you ever ride on them around 2018? Most of the seat cushions were completely worn out, some were positively painful to sit in. They didn't get much attention from Arriva, TfW had them refurbished to a decent standard.
The number of 197s stored out of use at Holyhead, Crewe, and Chester says otherwise.
That says nothing about the failure rate (how many miles a unit can manage on average before disrupting the service). Most of the reason that units are long-term out of traffic is that there are 77 units and one of the two main maintenance depots (Machynlleth) will only have the space to accommodate and maintain them once the 158s leave. Units parked up at the end of Chester Middle Yard for long periods tend to be the ones cannibalised to cover for hold ups with spare parts.
This is my concern. Why should Welsh ministers care about, for example, someone travelling between Hereford and Craven Arms?
Craven Arms is the interchange for Heart of Wales services. That function gets the station a much better service than a population of 2k would usually justify. A town of that size which didn't have the good fortune of being on a secondary mainline would have needed to plead exceptional hardship to dodge Beeching.
The 158s in particular were always a regular part of the Marches services, all the way back to Regional Railways days, before, during, and after the 175s.
And the 150s got use on the Marches throughout Arriva's stewardship, mostly to cycle them between their work in the North and their maintenance depot in Cardiff.
Had the lines survived, it's hard to see how any through services would have worked. They were never designed as through routes, with junctions that pointed the wrong way for through trains and routes that don't really go the right way.
If a through service existed it would only have done so for political reasons.
In practice if the missing links in the line were ever restored, I would imagine the services to consist of a loop (Birmingham-Bangor-Machynlleth-Birmingham), and either a self-contained Aberystwyth to Carmarthen service, or an extension of the Birmingham-Aberystwyth service through to Carmarthen.
So at the very least you'd still need to change at Dovey Junction.