Why do those 'accessibility' seat plans contain so much less information than
the detailed layout diagram for the 197s? Admittedly there is a lot of clutter on the latter that is unhelpful for passengers, but the position of window pillars and to-scale seat spacing (so you can get an idea of legroom) would be useful when booking seats. Although I note that TfW seem to have gone the opposite direction to LNER during COVID and stopped offering seat reservations altogether, they aren't the only operator to provide seat plans that are lacking useful information.
What ambience? As far as I can tell from the detailed seat plan (see link above), even first class does not have segregated vestibules (there appears to be a partion door between first and standard, but not between the first class saloons and the doorway). One of the bays in 1st is partially blocked by a window pillar too.
Seat pitch (spacing) on a 197 in airline seating is given on the diagram as 820mm. That is better than a 158 but less than a 175 (84cm by my tape measure) and even TfW have admitted that legroom is not quite as good as a 175.
2-car units are expected to remain the norm on the Cambrian lines west of Machynlleth. These can be quite busy particularly in summer. Admittedly the suituation may have changed now that more mark 4 sets are coming, but it also seemed likely that Milford Haven - Swansea would be reduced from the current mix of 2 and 3 coach 175s to 2-car on every single service (I think somebody said that the number of 197s fitted with first class would not be sufficient to both provide first class on all Holyhead-Cardiff services and allow the 3-car portion of the Manchesters to continue to Milford Haven).
There are also increased demands on the fleet though, in the form of the additonal Liverpool services (extension of Manchester Airport workings to Bangor might also require an extra diagram, as might filling in the last few gaps to go hourly to Aberystwyth?). Together with doubling up the Manchester-Swansea those extra services accounted for most of the 26 additonal units and, when you consider that there are three south Wales class 158 diagrams, the number of ETCS fitted units ordered (21) is no different to the current number of units available for the Cambrian.