"My wider concern is around who is actually going to take responsiblity for implementing robust policies for safety and wellbeing when it comes to abusive behaviour. If the denomination cannot insist — and it seems they cannot — then who will?"

That's certainly an imporant point, but I don't know there's a good answer. I can't imagine any practical, effective way to force independent churches to join hierarchal denominations. Not when hierarchy is religiously anathema to them.