It worries me that there are people who work as guards who appear to take the view that because some people - possibly lots of people - tell lies, everybody should be presumed to be lying. If staff advise and authorise use of a TPE train with an Avanti-only ticket during disruption then the passenger is entitled to follow that advice. If the passenger says staff authorised it then of course it is unreasonable to conclude that they are lying without taking any steps to try to establish if it's true.
While I agree it's a problem of staff attitudes, they are of course working for companies that often obsess over their revenue (not overall railway revenue), play ORCATS games, set TOC-restricted fares a few tens of pence cheaper than the Any Permitted, and so on. It is hardly surprising, if they've been influenced at all by the culture of the company they work for, that they would treat passengers with a TOC-restricted ticket for another company just as badly as one who hadn't bought a ticket at all. It's one of the reasons why, on balance, I would very much prefer the number of different operators on the National Rail network to be reduced to one.