If you are hoping that there is a definitive answer to this question (and it has certainly exercised a number of minds), then regrettably you will be disappointed.
Quotes from industry sources such as the final sentence of your extract from National Rail's website "All tickets are valid via any reasonable route unless specifically stated on the ticket." appear to offer some clarity and comfort, but of course by leaving "reasonable route" undefined, we still struggle to learn any answer from it.
The very reason why this question is so interesting is that every day, passengers in the UK are travelling without having consulted any definitive authority, and staff are inspecting tickets without having consulted any definitive authority, and yet only occasionally come across a dispute, and even more rarely, a dispute which requires a Legal Judgement which is binding on future travel.
It is very tempting to rely, as transportphoto and cacl7 suggest, on a printed itinery which removes some possibility of doubt about the authenticity of the ticket coupons, and that is good pragmatic advice for any passenger with uncertainty about the intended validity of a route. But that doesn't actually help us in determining the validity of the route. In fact, the documentation serves a different purpose, it demonstrates that the contracted itinery for that one journey had been issued by the industry-approved software and/or industry-employed ticket seller (and therefore should be honoured as a stand-alone contract); but both of those methods of issuing tickets make errors and both can be inconsistent, as we have seen.
That one-off printed itinery simply sidesteps the question of validity of the route for that, and for subsequent, journeys.
The Routing Guide and its instructions for its use go some way to establishing a procedure for answering the question, but as we've already seen, it contains ambiguities (e.g. direct trains on circular routes), incomprehensible statements and internal contradictions. The Easements too have dubious value (especially the Negative Easements which I commented on recently), particularly as they are not presented to the passenger in the same way as the more obvious and binding Terms. Its hard to see how the Guide can be definitive.
There has even been persuasive discussion here and elsewhere to suggest that some of the Conditions (obscure ones or otherwise) have been introduced without the requisite prior consultation which, if true, suggests that they may be unenforceable, even if widely known and accepted in regular custom and practice. But even if that is true, it doesn't clarify the status of those changes to the permitted and available routes during the time between their introduction and any future challenge to their propriety.
There's a popular view that the general effect of Consumer Legislation in dealing with unclear Contracts may help, but that is both unlikely (as there is no transfer of ownership in Rail travel) and no Legal Judgements from Consumer Legislation to refer to as relevant authorities. It's also often suggested that a Court would clearly draw one conclusion or other from a dispute over routing, but although there have been Prosecutions following off-route travel, none of those Judgements have, to my knowledge, given us any Precedent that assists us. No rational person makes decisions about the validity of an unclear Contract based on what they think a Court might in the future decide.
I expect that there are many posters on here who would like to provide helpful advice in response to your simple question, including myself, but I regret that there is not a simple and comprehensive answer, and I hope some of my reply helps to explain why my own unsatisfactory answer may be as good as it gets:
"Is it a permitted route?"
I don't know.