I'm not sure about the terms (and cost limitations) of TfW's tender but CAF units are known to be cheaper than Stadler so it isn't surprising larger numbers of CAF units were ordered. Stadler can build for just about any distance length and specification if need be, and have done around Europe. The Valley Lines are more of an S-bahn style service so the interior spec is just right for what they do and similar to what you get in Europe for that length of service. The 197s are more a jack of all trades design as they aren't just doing long distance but mid and shorter distance services as well, so are more like an updated Class 170 in that repsect.
A Stadler version of the 197 would be a 755, or a WINK variant.
The doors on the TfW Stadlers do look odd, it seems the Welsh still lack trust in Swiss single double door layouts being sufficient for dwell times. On GA single double doors work absolutely fine for all the services they run.
Vestibule doors are throughout the train, they just keep them permanently open most of the time, although the ambience is much nicer when they are closed.