I could sign a contract that says I could go out and kill someone. Doesn't mean that I won't be held liable for my actions if I followed through.

In my eyes if someone were to be injured or killed this would fall under gross negligence which would be a willful disregard for the safety of those in the audience. You wouldn't have a leg to stand on to defend yourself as the rules are well stated here and have been previously followed with regards to arena standards.

The event should have never taken place. If that is the arena that's been provided then you don't run the event until a proper arena is in place.