It is worth bearing in mind that platform staff often don't have any revenue training. Whilst they're working on the platforms they're most unlikely to ask absolutely everyone who asks any question at all about a train for their ticket in order to assess it's validity to qualify their answer.
To an extent the customer needs to be aware that if they have purchased a restrictive product, to which there is invariably at least some indication at the point of purchase, and that they may need to offer up evidence of it if they want to make a detailed query.
Otherwise you end with the likes of station cleaners or fitters having to ask for tickets every time they're asked a question for fear of their answer being held against them, which they might not even be able to interpret in the first place.
I always ask people who come out with "the person on the platform said it was this train" - "and did you show the person who said it was OK your ticket?" and the answer is almost always "oh no, I just asked if this was the train to X" or "no, I said my train is cancelled and they said this is the next one".
The circumstances here with Northern misleadingly indicating the destination of the train whilst still selling through tickets for the journey, along with a red herring cancellation do however make for a compelling argument that the matter should be dismissed, if you ask me. I'd not have issued the penalty in the first place, but hindsight is a wonderful thing and the inspector won't have had the facts neatly laid out for them with a bit of digging as we have.
I also think that often non revenue trained railway staff make the mistake of thinking that ticket inspectors are much more reasonable and forgiving than they often are, and that "it'll be OK" is genuinely the case - of course until it isn't.