I'm glad to hear that there has been a satisfactory outcome.
In case anyone is interested, I has a look when at Oxford station the other day, and the route the OP used appears to be completely unsignposted from inside the station, with nothing to either encourage or discourage its use as an exit. No doubt people do so use it, though -- I don't see why they shouldn't unless it's to avoid payment. I don't remember the 'West Oxford' sign that can be seen in post #6 -- either I was half asleep or it's been removed.
There is a poster on the right immediately inside the passage (about where the more distant poster on the left can be seen in the picture -- taken from the opposite direction -- in post #48), and probably fairly easy to miss if exiting with a crowd, saying 'Ticket Holder Entrance. This gate is open for ticket holders only at the following times:' and then giving the times. Further down on the left (roughly where there's a poster on the right in post #48) is one of those black and yellow posters warning about penalty fares.
Even if there had been a more obvious sign, it's not unreasonable to think that the presence of staff at a normally unstaffed exit might mean that tickets were in fact being sold there on the day in question, overriding the sign; approaching staff to ask where a ticket can be bought is entirely reasonable for someone in the position the OP they said they were in. FGW appear to have agreed with this (or at least realised that they'd make themselves look silly trying to argue otherwise in court).
If there's a problem with people fraudulently claiming to have boarded at Radley, then it seems that any sting aimed at them needs to include a staff presence at Radley to hand out tokens or sell tickets, so genuine Radley passengers can be distinguished from fare dodgers.