Tickets may generally be accepted on the ground in these cases, but you'll find that much less likely if the different routes are operated by different TOCs, because they see it as loss of revenue and it messes up their revenue distribution calculations.
Agreed. Where different TOCs operate different stations / routes / services, and / or the different stations are on completely different lines with services in different directions then the flexibility may be reduced. This case is generally OK as trains from both stations go to Leeds, and stop at the same stations on the same route anyway.
Another recently discussed example on here was Ash Vale and North Camp, which are only 0.5mile apart on a straight road and are in fact closer than Moorthorpe & South Elmsall. I wouldn't suggest tickets are interchangeable here as the stations are on different routes, going to different destinations, operated by different TOCs.
The only list of such interchangeable stations that I am aware of is with seasons tickets which are "also available at". These are mainly in the former NSE area, are generally for journeys to London, and have been around for a long time - I remember my father having such a season ticket when we lived there in the 1980s. In the case of Ash Vale, for example, Ash Vale has season tickets which are also available at Farnborough Main (not Reading) - the tickets are printed accordingly.