Not sure whether no-pull-ups will solve the wettig problem or not, but it
will help the issue of not having to step on soggy smelly pull-ups. If she doesn't change the sheets after she pees the bed, who suffers? She needs to have 'natural' consequences that will make sense in her world. Does she have friends over to play? Does she have sleepovers with friends? Those are good reasons to learn better hygeine. Perhaps encouraging that might be a step in that direction.
I completely understand when you say you resent her - it's not the her in that ethereal, spirit sense, but the her in the soggy smelly diapers on the floor sense! For her, though, does she pick up on that attitude and feel like an outsider? Does she need more than she can get from you (becuase you two are in a bad cycle!), and is she pushing you harder and harder to get it?
My guess is that what she's doing here is forcing you to choose between her bad behaviours and HER. If she believes that you will love her no matter what, if she gets that in it's entirety, she will probably start to work on these issues herself (which is the only way that she will get better about them - nothing you are asking for is all that hard lol!). Her seemingly lazy behaviour may really be a call, a pecking so to speak, so that you pay attention to her in ways that she wants (right now those seem like negative things - asking her to do things, expecting her to be better at stuff than she is etc.) - do you guys do dates together? Movies, shopping, bowling/arcade? Kids need to feeeeeel connected (not intellectually know that you are caring for her, but FEEL cared for). You two need to change tracks. Since you are the adult, you will have to do it first.
I recommend reading Children are from Heaven (by the Men are from Mars Women are from Venus fella), and also
How to Talk so your kids Listen
. Both of them should be in your local library system. They may have more ideas about how to speak with her so that she listens and respects you more. I HATE not being listened to - it would drive me nuts having to live with someone who so blatently disrespected my needs and rules.
I think you are doing an amazing thing, raising this child as your own. Now treat her as if she's your own - there is no where else for her to go. Clearly in her place getting to go live with a guardian who sets no boundaries would not do her justice. (on that note - can you set up a vacation time where she does go and live w/her g'dad? Would that be safe for her? Would he begin to understand how difficult she is?)
No matter what, she needs your unconditional (ie: even with her smelly diapers on the floor, sullen, know it all teen routine) love, your acceptance, and your patience. I can only imagine how incredibly trying and hard this is/will be for you both, but if you can start on a new path with her, it will at least be your very best effort at getting her to be a responsible person, a young woman with principles that you respect, etc.
As far as the 21 days goes? I dunno - maybe you and she can make up a chart of the things she needs to be doing every morning - if you do it together she will be clearer about what it is you expect in a way that isn't a shouting match kind of thing? Then you can set up rewards with her too - if she wants new music, she can get one CD after every 6 days of getting the stuff done you need her to do, and after 3 weeks of that, she can get her hair done? So that there are small, meaningful rewards to help her along the way, yk?
No matter how hard her background was, where she is now doesn't have to be hard, and she can and will learn how to do the things you expect ALL your children to do as young adults.

Hang in there mama - in a few short years you will hopefully look back on all this with a tremendous sigh of relief for having helped her through, learned so much about raising kids, and having survived teen-hood together.
hth?
andy