Experienced FPs! Should we hang on or let go? Wisdom needed!

First a brief background: While in the process of becoming placement ready, my husband and I simultaneously became foster youth mentors to a sibling set of 2 teens. Coincidentally, we became placement ready at about the same time the already unstable separate placements of these teens completely disintegrated. We were recommended as a non-relative foster placement for one of them. We had concerns that a lack of communication was the leading cause of disintegration of this teen's previous placement, so it was made clear to her that refusal to communicate with us was not going to be tolerated. The teen agreed, prior to placement, that she would cooperate with therapy (instead of just attending her individual sessions and refusing to talk as she had in the past) and would make an honest effort to communicate with us about how she felt and what she wanted/needed. She willingly and fully agreed to these terms. And everyone with a history with her said that we had connected with her and gotten her to open up to us more than she ever had to anyone. So we accepted the placement. Here we are 2 months later. At first things seemed to be going... well, definitely not perfect, but there was progress being made. We were optimistic that things were going to work out in the long run. Recently, however, after her first visit with friends from her previous placement (who live an hour away) she has done a complete 180. She is now severely homesick and adamant that she no longer wants to live with us because she wants to move back to the part of town she grew up in. I can sympathize with that. However, she is taking it farther and has become openly hostile and is treating us like her jailers. She is refusing to obey basic safety and common courtesy rules that weren't a problem before her visit "home" and is declaring that she will not talk about her feelings with us and is just shutting down & sulking whenever she gets upset, and she is now adamant that she will not cooperate with a therapist. We have also just discovered that she has not been completing her school work like she told us she was and is now in danger of not passing the 8th grade if she doesn't complete a certain amount of work in the next 4 days and she is STILL refusing to allow us to help her with her assignments. She says she just "can't wait to finish school because then [she is] going to run away". Not because she doesn't like living with us (although she now constantly complains about the "stupid rules"), but because she wants to be in the other area of town where things are familiar, where her "friends" live, and where she thinks she will have more "freedom." The problem is that her previous placements have all been kinship placements that basically let her come and go as she pleased at all hours of the day and night for whatever length of time she so felt like. They didn't keep track of her education except to punish her when her report cards came with Ds and Fs. She was flunking half her classes and had transferred schools twice this year before coming to us. Until this week, she has always claimed they didn't push her to talk about how she felt. Now she is saying that when she first got there she made the same agreement to communicate with them that she made with us, but she didn't want to then and she isn't going to now. In the past she has told us that when she actually told them what she felt, they usually responded with anger and refused to change anything they were doing. She knows, and has admitted, that she doesn't get that response here - that we react calmly and openly discuss things with her - and she was making slow progress toward opening up to us more and more until she came back from the visit with her former friends. Now it's like she got a taste of her former life - her comfort zone- and is bound and determined to make our life a living hell until we cave and let her go "home." Except there isn't a real option for her to do that - despite what she thinks. There are no families or group homes with a current vacancy in the area she wants to move back to, but she swears her CW is just lying about that. Add to this that she has BMom encouraging her via phone calls to ask her CW if she can go live with her. We already know that that is never going to happen, and our FD doesn't even really want it to happen - she knows her mom isn't really able to care for her - but the mom's repeated urging causes her to second guess herself and gets her upset with the CW and just muddles things up even more. Then she has her sibling telling her that she should move to a group home (like the one sibling is in) because "you get so much more freedom in a group home." yada yada yada ... And now FD is saying she is going to ask her lawyer to get her moved back "home" even if it's to a group home because, of course, CW is telling her no she can't move. CW wants her to stay with us. Even her former kinship placement (whom she does NOT want to go back to) wants her to stay with us. The only ones who don't want her to stay with us are her sibling (with whom our mentor/mentee relationship has deteriorated - though not ended - as a result of circumstances beyond our control) and her BMom. We really wanted her to stay here permanently. We wanted to work through her issues with her and have this be her permanent home where she came back to from college. That was the plan. That was what she said she wanted. That was what was on the placement papers. That was what we all planned for. We and her CW were even discussing the possibility of guardianship in 6mths. BUT... we can't force her to participate with our family life. We can't force her to care about her education. We can't force her to tell us how she is feeling or why. We can't force her to cooperate with therapy. We can't force her to obey the basic safety and common courtesy rules. And we can't stop her from running away. Meanwhile we have young biochildren who have bonded with her and will be seriously hurt if she chooses to leave. Children who were in tears over their fears of bonding with her and being rejected, but whom we convinced to give her a chance. And the drama she is creating is tearing us all apart. EVERYTHING is a fight with her now. She argues about needing to eat 3 meals a day. She argues about being left alone - as in she wants to be home alone or stay in the car while we shop. She argues about us needing to check her schoolwork. She argues about playing her music too loud. She argues about wanting to go places without adult supervision. You name it, she probably argues about it now. And it's not always a long drawn out argument. As often as possible, we try to nip these things in the bud - try to not engage - but it is just constant like dripping water and we are wearing down. And although some of these things may seem like "typical teenage issues" they weren't happening until she visited her friends and decided she didn't want to be here anymore. She has just completely stopped trying to make this work. She has admitted that she is deliberately making our lives hell because she wants us to kick her out. In total, we are spending about 3-5hrs EVERY DAY talking with her about her attitude and the choices she is making. Then we go to our room and spend another 3 or more hours talking with each other trying to figure out what in the world to do. And at this point we are wondering if we should just give notice and let her leave? Because if this keeps on .... I don't know how much more our marriage, our family, our sanity, can handle. But she has nowhere else to go. She will wind up languishing at the intake center, waiting for a group home bed to open up or another unsuspecting family to take her in. And I say unsuspecting because she will never be able to make it in a family setting if she doesn't learn to communicate and she is now adamant that she is unwilling to do so. I don't want to give up on her. I believe that languishing in the intake center or moving to a group home would be like a death knell for her future, but I also worry that it's only going to get worse if she stays and that it could just grind us all down until it 1) removes any possibility of us being able to retain any positive role in her life when she ultimately moves on and/or 2) destroys our previously strong marriage just from the sheer stress of trying to handle the constant conflict and hostility coming from her, nevermind what it will do to our biochildren who have to live in this environment and deal with perpetually burnt out parents. I'm hoping someone reading this can relate and advise. Do we hang on or let go?