Showgrounds services could operate using only the Belair line with turnback at Millswood loop.
If the station was fit for public use (which it won't be in time), what would be wrong with shuttle services turning back at Kesville station if shuttle services did happen? Terminating at an intermediate station on a single track without a pile of gravel across the line is now routine operation, it happens on a daily basis at Osborne.
In any case, the only show-specific shuttles I think we're going to see this year will be buses to the nice large off-street bus stop reservation on the Goodwood Road side right next to entrance gates. Running rail shuttles to Goodwood station would be rightly condemned as a non-service and the word would get around within the first few hours of operation that using the buses is better this year.
If the system can get sorted out enough to provide the promised four trains an hour each way to/from Tonsley while maintaining two per hour to/from Belair, I can see there being no need for dedicated shuttle trains during the day
when the regular timetables can handle the load, especially if all Seaford trains were to have Waywick stops added to spread it out and Belair trains were strengthened to three cars each.
Having a handful of shuttles at night only
after the end of an event (not just the Royal Show, any large event there) could be useful to make up for the lower frequency of the regular timetables at night and to provide a better arrangement for crowd control. Best practice would be to run Belair trains through there on the Seaford tracks so that full-height barriers at the entrance to the bridge can control access to the shuttle platform, letting nobody in for timetabled city-bound trains (effectively making them set down only) and 1000 people in each time a two unit shuttle arrives. Once the crowd waiting drops below 100 people, the barriers would be dismantled and the shuttle service called off.