I think there's a general issue about advice given by staff at stations.
It seems that two sets of staff at KX managed to give the OP wrong information. Probably because they did not look closely enough at the ticket held.
Should there be some kind of protocol for staff in these situations? For example, should there be specific training not to advise anyone to take a specific train unless the ticket has been properly examined? There is not much use having staff to hand if the advice they give can't be relied upon.
Maybe there are times when a member of staff doesn't have the time or knowledge to examine the ticket fully - and maybe in these cases they need to clearly say "sorry, I can't advise you on your ticket validity - please check with X or Y".
This case actually reminds me of something that happened to me at KX not that long ago: I had a Lumo ticket to Edinburgh, and there was major disruption on the day, such that the Lumo train previous to my booked one was actually leaving after my scheduled departure time. I wondered if they might let me get on that one - I was let through the gates on that basis, but I also then asked someone in a Lumo uniform, standing next to the train, if it would be ok for me to get on that one even though it didn't match my ticket. Yes that's fine she said, only saying that they couldn't guarantee I'd get a seat, a risk I was happy to take. A little while after departure the conductor came round and looked at my ticket and said I was on the wrong train. I explained the advice I'd been given on the platform and he frowned and said that's not right and who exactly told you that. He did agree that I could stay on the train, but clearly only at his (slightly reluctant) discretion. I was OK in that case but it did make me realise, if you are given advice verbally by any member of staff you actually can't rely on it at all, because if you're subsequently challenged you've no evidence of anything.
(In fact, on that occasion I think I saved Lumo quite a lot of money because had I waited for my correct train, it would have got to Edinburgh so late that I'd have missed the last train of the day for my onward connection and I would have been making claims for overnight accommodation and possibly new Scotrail tickets the next day)