Pwllheli - Machynleth is up 25 stops in 57 miles and, I gather, very crowded in the summer.
Merthyr Tydfil - Bridgend via Barry is 23 stops in 50 miles and very crowded at peak times.
Ignoring things like gauging issues, are 175s suitable for the former, the latter, neither or both?
Neither. The Pwllheli services normally run through to Birmingham/Shrewsbury as a portion of the Aberystwyth services for which end gangways would be very desirable (and if it were up to me would be made mandatory in the franchise specification). Compared to a 158 the 175s are also rather heavy / expensive for a rural line like the Cambrian.
If I put aside the portion working and assume Pwllheli - Machynlleth was run as a standalone service the lack of gangways would no longer be an issue but the weight/cost issue would remain and the top-speed would then be rather excessive. Something geared for faster acceleration and a 75mph top speed would then be more-appropriate. This service would not include a major city so there is no heavy peak commuting load - I assume therefore that the crowded services (except perhaps the 'school bus' rail service, I don't know how full of pupils these are) are crowded with leisure travellers and not commuters. Where the services run through to Shrewsbury/Birmingham there a commuting peak but in these areas the Cambrian services are limited-stop (either by missing out the smaller stations east of SHR or by virtue of the fact that, other than Caersws, there are no local stations between Machynlleth and Shrewsbury) so passengers are generally making journeys significantly longer than 5 minutes.
I also assume (though have no idea how to prove it) that the peak-time valleys services are significantly busier near Cardiff than at the extremities, whereas the crowded Cambrian Coast services are relatively evenly loaded (ie. busy) throughout the trip and (combined with the higher proportion of leisure passengers) therefore have a stronger case for strengthening throughout the trip to avoid standees. Similarly, I assume passengers are far more likely to be on one of the Cambrian services for much more than an hour than on the valleys - Merthyr-Bridgend is quicker if you change at Cardiff Central and the vast majority of passengers on that route are probably heading to/from Cardiff.
The problem with the narrower doors, is that it gives problems for people that are disabled, especially those in a wheelchair to be able to get off the train even with help from the guard.
You have to remember that not everyone is the same width in body nowadays, especially with generations eating foods from international fast food brands. Those sort of people can due to problems they suffer, can still end up in a wheelchair or having to walk with a stick or frame. Think of someone like Geoff Capes in a wheelchair and then maybe you can understand, how your suggestion goes against people that are disabled.
We have PRM leglislation now which I assume sets a minimum door width that wheelchairs will fit through. Ok; the 158s may not strictly comply with that but the 175s, 444s, 222s, 390s, 397s etc. (being significantly newer) presumably do and still have narrower doors than the worryingly ubiquitous 1300mm outer-suburban double-door. There is a wide range of disabilities too - for example some people may need more-frequent access to a toilet so I suppose it's a trade-off. You could have wide doors to help wheelchair users board but then there is less space for toilets (or other things) that can help with the rest of the journey. Once the passengers are on-board and the train is moving, the vestibles are effectively wasted space until the next station. I had to Google who Geoff Capes was, so we may be talking about different people, but Google says he is 1.97m tall - I'm sure someone that tall (who is not disabled) would appreciate the seat-pitch (84cm by my tape measure, would be great if somebody can confirm this) in the airline-style seating on a class 175 - I believe this is superior to nearly every other UK train in standard class. I'm over 6ft tall myself and when I was on the class 150 ATW provided for the Fishguard Boat train (an extreme example maybe, with only 4 or 5 stops) my knees would be crushed against the seat in front and I'd be resenting that all that space around the doors like hell - let my knees have some of that space.
I just don't see dwell times as being much of a problem where the journey is much over an hour, or the stops are much over 5 minutes apart or (as on the Cambrian Coast or Heart Of Wales Line) only lightly used. Even a 158 doesn't spend very long at a stop if only 4 or 5 people are getting on, and they have doors which (as well as being narrow and at the ends) appear to be rather slow-to-release. Passenger comfort (to provide a nicer environment than a private car to encourage modal shift) is far more important in my view - passengers having to stand is no good for modal shift.
