You like playing devil’s advocate from time to time and so do I!

I believe the answers to be "no" and "no". And the
DfT» know it.
If everything had been followed correctly, we wouldn't have had to wait two cycles for the
FOI▸ response. And if the outcome had been fair and reasonable, we would not have such support not only from the public, but from the expert too.
When I was Formal Complaints Officer for a housing association (internal Ombudsman if you like) who prided himself in solving complaints well before they got anywhere near the Housing Ombudsman, I used to ask two questions:
1. Were all policies and procedures correctly followed?
2. Even if they were, was the outcome fair and reasonable under the circumstances?
Having answers "no" and "no", Robin, much of the rest of your case is somewhat built on sand, and is very selective in what and how it is presented.
I think the conclusion we're coming to is that there is no-one charged with holding the decisions of the DfT to task, even where hardly anyone agrees with them or their messages. The Rail Ombudsman and Transport Focus are both charged with taking care of the farthings and mandated to ignore the guineas.
[snip]
So in truth, your complaint boils down to a lack of convenience both in terms of having to change trains and having poor connections (call it poor customer service of you like) but even so the matter leans towards the subjective. If you now have to change trains on what was once a through journey, any connection could be seen by some to be a poor connection.
Could - but isn't. Take a look at the poll I ran, Robin , to check the validity of the connection offered. Only 1 in 40 felt that over 45 minutes was reasonable. Whereas almost everyone is happy with up to 15 minutes. And people make choices on how they feel and what you describe as "subjective" becomes objective.
As regards changing trains and poor customer service, if, as I understand it, SWR» want the stock to augment capacity on the Salisbury to Exeter route, they may argue that your resultant reduced service is more than outweighed by an improvement in service for others between Salisbury and Exeter. And an official body might well agree with them.
Might ... but none has they have gone the other way
In addition of course, SWT▸ are one class 159 short at the moment as a result of the mishap at Salisbury. Now might not be a very good time to argue for more coaching stock to be in service for that reason alone.
It does indeed give them a soundbite to use against those who haven't checked up on the details, read the FOI, or thought it through. How useful for the DfT that most passengers all into that uninformed group - remember that you can fool all of the people some of the time, and some of the people all of the time.