So is the arena portion 200x75 or less the RP more like 125x75? The problem I have is where the curved area of the RP and the fence of the arena come together. Any tight chute area like that is asking for trouble.
What I would do is install 2 16 gates on the RP (both sides) where the curve meets the straight fence line. You can chain them open and use the entire area to ride in giving yourself a much larger area or you close them up when you need to use the RP. It will also enable you to get into the RP with a tractor to harrow the footing. You will also want to be able to swing one set of panels on the portion that is disconnected in towards the RP to give yourself a larger opening and to stabilize the fence.