Wow, that's a mess. Sounds similar to a problem I had once on an "open ride" night where there wasn't supposed to be barrel practice (they had other nights for barrel racers to practice) and a girl that was "working the rail" was going one way one second, then turned her horse on his hindquarters and slammed right into me and my friend who were riding the rail at a walk. No serious injuries but my mare did kick at the other horse.
Is there really nowhere else to ride? If there is a field or cleared area with decent footing (not covered in tons and tons of rock like it is here in the desert) maybe you could start riding there? If the horse is even relatively safe and well trained you shouldn't really need a fenced arena.
"You can do something wrong for thirty years and call yourself experienced, you can do something right for a week and experience more than someone who spent thirty years doing the wrong thing." ~WhattaTroublemaker