Hi Snools33. Firstly I think you need to take a deep breath and give yourself a massive pat on the back for all you do everyday for her - you sounds like a great and caring Mum and it can be really hard for us not to beat ourselves up when challenges like this come along.
Secondly I have a fair bit of experience with 4 teenage girls 13,15, 17, 19 (!!) and here is my tuppence worth!
Although i do understand your urge to check her phone and it is good you have found all this out, I would start to be really mindful of this tactic from now on as she gets older. If she found out in any way you were checking her phone she would feel her trust in you was broken, and believe me this could be a nightmare. It is all about starting to build trust. believe me it's hard to let go but its important as they sense when you are holding the reigns to tight and it makes them tug harder!
The face scribble thing is their way of staying "safe" so that their identity is private even if photos are - there's a misguided logic in it!
I think you can do no harm in continuing to boost her inner self esteem, offer her genuine and sincere compliments on her actions / behaviour / character as often as you can. 'Catch her' being good and tell her you've noticed how kindly she did this, or thank her for the thoughtful way she did that. Even if she shrugs off compliments she will hear them deep down.
Don't worry - the obsession with materialistic 'things' and looks is practically universal with teenage girls!
One thing - I firmly believe that teenagers rebel against their parents's way - this is very common. So if your lifestyle is very non-materialistic it is actually not a surprise that she wants to pull away from that and try something new. Don't fight it too much as it will push her further down that road. Keep the family grounded but accept and try not to judge her interest in trivial things as no doubt we all had as teenagers!
An allowance is a great idea. Our 4 were given pre-payment cards (such as Optimum or Splash). These look like credit cards but really they are no such thing. You the parent can load the card up with cash in advance, each month etc and she can spend it like a real card. You can both keep track of her spending by logging into an account and there is also an app showing the balance and spending. These cards are especially designed for teens so there is no impact on any credit rating or anything like that - to all intents and purposes it is cash. We found this was brilliant as it was essentially giving pocket money / cash, but made the girls feel more responsible / adult and supported them to start budgeting and managing money.
With hobbies I would try not to push or even actively encourage her down any path as she may well resist, but you could provide a full range of options and make it seem as if you have no preference. Something one of my daughters did was a youth theatre, which she loved and it really boosted her confidence. Also try looking for local choirs, art clubs, dancing, sports etc and offering any of these up as a reward.
Finally, to encourage her to think about managing her privacy online generally you could draw her attention to the current news story about Facebook stealing data and how it may have influenced democracy. You never know, she could be interested!
I hope this helps and good luck!! xx