No ones exasperated with you, we have our own teens to properly exasperate us! 
Totally agree you and your DH need to be in general agreement, and this will take discussion.
I've just gone back and read all your posts without reading the replies if that makes sense. It seems, as a bystander, your main problem is not that she is doing anything terrible, ie stealing, drugs, truancy - this is good news. BUT there is conflict, the atmosphere in the house is making everyone, especially you, upset and fed up.
Maybe if you sat down with your DH, agreed your three major problems (for example, curfew breaches, arguing and excessive phone use) Then you and your DH think of solutions to solve this behaviour and AGREE which ones to adopt. I know this will be hard, it's easy for me to say it! but maybe think of it as a work meeting, each be polite, listen to each other and make it clear at the end of the discussion you need to have an agreement.
Then sit down with dd, explain how this is making you feel and you have been discussing ways to make it better for everyone. Say you have some ideas, you are going to trial them and she HAS to give this a chance and abide by it.
Rather than you and DH sitting down with her where she might feel ganged up on and it may be easy for you and DH to be conquered and divided
, if you drive, can one of you have this conversation in the car (tip I got from MN, works very well with my DC) and then just discuss it calmly at home when hopefully dd thinks you are being fair and feels she is being listened to - IME, crucial for teens.
I don't know if it's useful or not for you. To be very honest, she sounds pretty good to me compared to some teens I know, and this is a dangerous age to alienate your DC. So the advice I'm giving, isn't really specific with solutions to your issues, more useful advice I learnt (mainly from MN!) and hopefully it will help a bit.
Teens require different handling. They make toddlers look like a walk in the park sometimes,and the methods we used to use don't work any more! My worry is, if DC feel nothing they can do is ever good enough, they can just stop trying all together and with her GCSES coming up, it wouldn't be worth risking this IMO.
Think long term too. My DH doesn't see his parents any more (much more serious issues and you sound completely different) but still they've missed out on their gc, and have lost their own DC which is sad. So to put it into perspective, you are by no means a bad parent, you want the best for your dd and it's important she realises this!