if your daughter is know to just diappear for days at a time and safety is your primary concern, i think authoritarian is the wrong way to play this.
i think you need to be her confidant, forgiving, eager t meet boyfriend - find out what he likes with regard to football or something find a commonality even if its ribbing each other about how rubbish each opthers football team is.
you need a conversation with your ex regarding contraception - or if that wont work - with your dd.
i think you need to listen and withold judgement and advice - and i am going to be completely sexist now and say that i think on the whole men like to solve a problem that they hear
men like to answer questions that arise.
i think you need to just make affirmative listening head nods etc. i can highly recommend having conversations in teh kitchen when you are doing something else - they can sit on the kitchen side and you can turn away and bite your lip whilst pretending to put the dishwasher on.
you will hear things you dont want to - but i always think its better to know than not know.
i think its about measured control, so she might think that she is going to her boyfriends with your approval - but actually what happened is this
you drove her there
you know where she is
you know where he lives
you know when and where you are picking her up
she is as safe as you could have possibly made her within these circumstances
even though you want to chain her in the basement
hope this helps my friend