No, it's perfectly valid as a point. I had it last year on a service from Pembroke Dock owing to the staff member being caught up in disruption, not that "whether it happens often" matters.
The point is, you have no grounds to suggest RTT or the public timetable is authoritative. Both say all sorts of things about train services, and whether a "bonus" trolley or "bonus" first class carriage appears doesn't mean you can use either for free. And even where the data is restrictive, like saying reservations are compulsory, they in fact are usually not!
The passenger contract is really clear about what you're entitled to, and it's up to GWR to say otherwise. And they haven't, so far, and the guard was within their right to state that the first class carriage was marked and it wasn't declassified.
Not relevant even if true (it isn't, I note!). I can buy a first class ticket from Rugby (or many, many origins further afield) to Weybridge, that a ticket can't be sold a first class for a particular flow doesn't mean the trains you use de facto "have no first class". What matter is whether the TOC tells you it's declassified. And I expect they usually do on that route; I used to use the stoppers from Woking into Waterloo and a Desiro would occasionally work the service; SWT (as it then was) declassified with announcements and notices.
Of course it can, if the TOC or guard declassifies it. As is often the case.
Well, sit in standard class then?