about controls - yes, there are controls, you can also read that in the "Forum ABC". I´ve seen them at the small streets (not the highway) quite near to the festival. But luckily they never controlled my car...
about violence - if you don´t have a "hippie mobile home" with dolphins painted on it, nobody should attack you
A friend of mine who had such a car really got annoyance with some stupid intolerant guys some years ago... I´m still wondering about that story... but that should be an exception, I never experienced any violence at Wacken