Statistically you will get caught in the end with these railcard ruses. They are so prevalent nowadays. I missed my train to Greenwich the other day because the gate didn't open and someone then wanted to see my railcard on my phone. I probably get asked, on average, once on the train every ten trips. If you are that worried now about something that I don't think they care about, I don't see how it is worth going through the stress of the whole travelling procedure knowing you have an invalid ticket. Just pay the extra to make your journey less stressful.
I'd say they don't care - because no statement was taken. They contact you weeks later through some tracing system, what are you going to say? You came through the gate with a valid ticket - just because it wasn't scanned in - who cares? Unless they are using timing points to note you only bought the ticket just before showing it. But they'd really want to catch you to do that because you could still argue you showed the wrong ticket. As long as you have nothing else to hide, I'd say you would be OK.