One other thing I have wondered - but not being a Gloucestershire person this may be silly - could the train headed up the Stroud Valley call at Gloucester just once - i.e. Stonehouse, Gloucester, Cheltenham Spa, Stonehouse one hour and Stonehouse, Cheltenham Spa, Gloucester, Stonehouse the next? Are there enough other trains between Gloucester and Cheltenham Spa for this to work?
Trains can by-pass Gloucester easily, but Gloucester is a significant traffic generator so it might not be sensible.
Is there a crossover south of Cheltenham Spa station that would allow it?
The crossover is just north of Cheltenham Spa station where the turnback siding is.