I don't think any Four wheel truck is suitable for journeys over 30 minutes, I don't think that a 142 or 143 is really suitable for Exeter To Barnstaple let alone Exmouth to Barnstaple.
But I seem to recall in the Wales & Borders days there was a class 153 working from Milford Haven to Penzance, think it joined something else at Bristol to work down. Might even have been the other way round with a PNZ to Milford Haven train.
The options had the 142's not been subleased and put into service on the Devon internals were limited. Buses to Barnstaple would probably have been the order of the day along with reducing the Exmouth line services to the franchise minimum of hourly. So better to have 142's than no trains at all I would have thought.
You are quite correct about a class 153 working through from Penzance to Milford Haven, this was worked as a portion off the single through 158 service in each direction from Penzance to Manchester. The 153 detached from 1M61 at Bristol
TM▸ (1M61 didnt call at Newport and ran round Maindee East curve, right away to Abergavenny). The corresponding return 153 working attached to 1V79 at Newport for the journey onward to Penzance.