6. I think is the "easy" one - none is on the national network, as isn't Birkhill neither.
I got sidetracked by the fact that the first three also have have a bit tacked on the end of their original name (with something else left off in one case), but there never was an Irwell station, was there?
Yes, stations but NOT on the National Network was what I had in mind.
No 5. Might be that the stations had originally a signal box incorporated in the design that was in an elevated position?..
That was not the connection I had - and I don't have a background that lets me easily confirm or deny your assertion. As a clue to what I had as commonality - look to services rather than to structure / the bricks and mortar.