The situation sounds very stressful. It also sounds to me as if you are trying to take some 'blame' yourself for your early parenting, and so are still trying to make amends. This doesn't make sense to me.
It's easier from the outside than when you are in the situation, but personally, I'd stop running around after her, giving her money, groceries or anything else. If she loses clothes because she doesn't have the money to pay for them - that should be her look-out. She knows the system, but knows that you will bail her out, so where is the motivation to be independent? She's not a baby - wow, many girls are raising their own children at seventeen, not being the baby themselves!
I think that the only way you can help her is to stop helping her to be helpless. I'd be clear to her, have a talk, tell her she's made her choice and is now out in the world. Let her fend for herself - you'll be doing her a big favour in the long run. Let her decide what to do about school. My hunch is that the more you chase her to go, the less inclined she will be to do so. And while she thinks that you will support her financially, there is little motivation to look after herself and her future.
I don't know if I'd bother to make 'deals' - I don't think the idea of reward and punishment works in the long term. I'd tell her that she's welcome to come home to dinner x times a week, and visit, just as any normal daughter would, but that the gravy train ends here.
I hope I don't sound too harsh, I am not meaning to be, but I agree with Khris that you are enabling her to be lazy and helpless. It sounds to me that you need to break a habit of a lifetime and it will take strength on your part to do it.
Good luck, it sounds like a very frustrating situation!