I am struggling to see why you are creating an issue over getting to the Triangle. We live in Stanford in the Vale, not much more than one bus an hour to Faringdon and change for a connection to Oxford. We are planning on driving, as we do now, to games. The difference will be that we will be able to drive straight to the Water Eaton park & ride and either, walk to the ground or get the shuttle that will be running between the two. We will arrive earlier and enjoy the bars, restaurants etc located around the ground. On days when the weather is favourable, we will take something to eat with us and enjoy the green plaza (as indicated on the artists impressions) On leaving, we expect departure from the park & ride to be relatively simple as the exit will be traffic controlled and planned to facilitate easy movement. There will be far less people using their vehicles as there are trains, buses, cycle paths etc all making life easier. We have even deliberated about using one of the other park & ride sites and going into town before or after the game. Rather shamefully, I must admit that we have only recently used the park & ride system to go into town. It was a pleasant surprise to find the parking easy, very cheap and the bus service frequent and clean.
From your area, West Oxfordshire, I would expect that the, soon to be built, park & ride off the A40 would be perfect, if you did not wish to go all the way to Water Eaton. Buses run between all the sites and I would not be surprised to see the club organising it's own shuttle service to accommodate.