I think it's understandable that you are concerned.
Her behaviour imho isn't quite "off the rails" but it's certainly heading in that direction if she decides to up the ante.
Upshot is she's engaging in teen rebellion and the more you try and exert your control the more she'll push back and escalate her behaviour to thwart you.
I think you need to change tactics.
She wants to be treated like an adult and be allowed to make her own life choices.
My suggestion would be to sit down and talk with her (when you are both calm) and work out a new path forward.
For example, if she wants to smoke (knowing the dangers) then ok, but not in your house/garden, nor are you going to fund the purchase of cigarettes. So if she makes that choice, she needs to get a part time job to fund that habit.
She can have her phone back, you won't snoop but ask her to consider how she would feel if those pictures became public. She'll deny it will ever be a problem and you say you hope she's right, but thousands of other girls have found out they were wrong. She's taking a risk and you hope it doesn't backfire on her but you'll respect her privacy going forward.
You might also want to think about any other concessions you would allow that would be in tune with her being treated like an adult. For example bedtimes, curfews, clothing etc. Try and show some willing here.
Re: school I think this is were you can "pick your battle". Whilst she's living with you some rules aren't flexible. If she's in education then she commits to it. If she keeps skipping school, failing exams then she's wasting her time and your support. If she doesn't want to engage in education then she needs to leave, get a job and start paying her way by contributing to the household. It's her choice, and adult choice - contribute by getting an education or getting a job and paying board.
Finally don't speak to the boyfriends family. Your child's welfare isn't their concern and frankly if you can't stop her taking compromising pictures why should they be responsible for deleting the evidence (and in doing so invading their child's privacy)?
This is between you and your DD and I think you need to back off a bit before you push her so far into a corner that she comes out fighting and really escalating her behaviour.