Though of course the acceleration of 170s is such that they are not suited to stopping services...
That's what I've read in the past which led me to the conclusion that 170s, like Voyagers, were a flawed design that isn't really ideal for anything.
I honestly don't get the obsession with avoiding 1/3 2/3 doors.
Well, everyone is saying they provide more standing room. Obviously we don't have TARDIS technology so logically that extra standing room must be at the expense of seats, or legroom, or tables or a combination of these. I personally don't think it acceptable to expect passengers to stand for more than about 5 minutes, except perhaps in the high-peak. Therefore I don't see increasing standing room at the expense of comfort on long-distance (semi-)fast services acceptable.
The other reason is we once sent one of my brothers (not an enthusiast like me) by train from Port Talbot to Nottingham. Oh his return, he said of the Cardiff-Nottingham train "The doors were in the wrong place".
I think you are being far too prescriptive in the five types of route that you are splitting things up into. The real railway is a lot messier. A 125mph service from London becomes a (virtually) "all stops" service as it gets closer to Penzance/ Aberdeen/ Hereford etc.
Messier yes, but I reckon those five types of train cover most bases quite well. The Great Western services you indicate seem to use the stock suitable for the part of the run which requires the highest-standard.
What is Manchester - Pembroke Dock, for example? Fast from Manchester to Crewe, but every stop beyond Carmarthen.
That's regional express. To me at least, the 'regional' bit means the service can be all-stops on sections of route where there is little or nothing in the way of other services, and the 'express' bit means it spends some time running (semi-)fast on a busier stretch of railway. I've seen somebody on here describe 158s as a 'Jack Of All Trades, Master Of None' in the past. Personally, given the diverse nature of regional express work I think a Jack Of All Trades is actually Master Of Regional Express.
Lastly, spend some time on a busy platform like 13/14 at Manchester Piccadilly and note the way that end-door stock stock like 158s takes a lot longer to unload/ fill than something like a 170/185.
Not the first time I've heard that one... As such, when I left the Manchester area I timed the dwell of the 175 I took to Chester. Can't remember the time now, it was months ago, but the dwell was actually similar to the 185s, and faster than the 158 turnrounds in Aberystwyth. I expect that was down to the 175 being empty when it arrived on the platform, but I'd guess the dwell with passengers to alight as well as board could be similar by spreading the passengers over a 5-car 175 rather than 3-car.
Reluctant as I am to take this thread more off topic, I agree with Rhydgaled. Now, what are the chances of getting back to transpenine 350s?
Yes, sorry, off-topic. Although, I think it was said earlier in the topic that the TPE 350s were a temporary measure to make use of the north-west OHLE until the full TPE north wiring scheme is implemented. Perhaps then suitable stock can be introduced (the oft-called-for 25KV AC version of the 444/442).