I understand your worry so I got an idea that could help you. Have a disclosure agreement to members (and even to non-member if posable) to mark that they are responsible for the content they put onto their channels. that way if there is any legal problem you can show the member responsible has taken the media from a problematic source.
a lot of companies do this to protect their website while allowing a wider freedom . You can even make this a member’s only feature just so you can tie it down to a person responsible .