It’s really a loophole. There is no genuine reason to do the double back.
There may be no genuine reason for a passenger to Stechford to double back at New Street - but I strongly object to customers having their contractual rights infringed, potentially in a way that itself constitutes a criminal offence. The fare differential is clearly a product of the privatised railway; it is intolerable for the privatised railway to reduce passengers' rights which are supposed to be historically protected as a condition of privatisation, merely because they want to rip off certain customers more than others!
Sorry but a ticket from Northampton to Stechford would expire as soon as the train calls at Stechford. It will have no further validity after calling at Stechford
That may be the way it is intended and priced, but fares regulation prevents it from being that way! It is unquestionably valid via Birmingham and anyone who denies that clearly doesn't understand the application of the Routeing Guide in this situation.
even if you have a dodgy iternary from NRE showing it is somehow meant to be valid by over travelling and doubling back via New Street
The Routeing Guide and the NRCoT encourage passengers to check an intended route on NRE if they are unsure whether it is a permitted route. If a through ticket is suggested in conjunction with an itinerary on NRE then it is deemed a permitted route. There are a variety of laws and rights which dictate that what NRE says is effectively and contractually binding. There can be no such thing as a "dodgy" route - if NRE offers it in conjunction with one ticket then it is valid, end of!
I dare you to try it, and challenge a RPI.
It is not difficult for me to take up your dare; Stechford is only a few changes away for me. I am very much inclined to try it out, though the chances of encountering an RPI are sadly miniscule (throughout 7 years of commuting on WMT and other TOCs I have never encountered an RPI onboard their trains, and only ever 3 or 4 times at stations).
They will take one look at the Northampton - Stechford ticket, knowing that Stechford is before New Street and decide you have overtravelled
They can "decide" what they like, but they would be very wrong to come to that conclusion. If they are so badly trained that they can't apply the Routeing Guide properly then they should not be in their job.
do you fancy your day in court in front of a judge
It would never even get that far; there isn't a chance in hell I would give my details to any RPI who claimed that this was an invalid route and that I had overtravelled! I would have no qualms explaining to them exactly why they were wrong.
who will ask "if your journey was from Northampton to Stechford, why didn't you alight from the train when it stopped at Stechford?"
Your journey does not have to be the same as what your tickets are to and from; virtually all walk-up tickets permit starting late and/or finishing short. I would be quite honest in explaining my journey was to/from New Street. There would be no reason to get off at Stechford if my journey is to New Street. If they questioned the route via Birmingham I am sure it would be possible to arrange for relevant expert witnesses from this forum to verify that the route is valid, if my own explanation doesn't suffice!
It is like being able to double back from Milton Keynes on a London - Bletchley ticket when the train you use from Euston to Milton Keynes calls at Bletchley
You can't, because there is no doubling back easement and Bletchley is its own Routeing Point and the only mapped route to London is direct, not via Milton Keynes.
Or on a Salisbury - Romsey ticket deciding to remain on board a Salisbury - Romsey figure of 6 service after it calls at Romsey and trying to exit the station at Southampton with someone claiming it has further validity via Chandlers Ford
That is quite arguably permitted if you are taking through trains between Salisbury and Romsey at all times (there have previous been threads about whether you can break your journey on a route which is permitted only by virtue of being on a through train, the conclusion being that the NRCoT and Routeing Guide are ambiguous on this point and thus Section 69(1) of the Consumer Rights Act 2015 means it is permitted). Obviously the requirement of using Salisbury-Romsey through trains at all times limits your choice of services quite substantially.