Oh Doin, sorry it's all kicked off again. :(
I'm afraid you are in very difficult position. I'm afraid because your DD is nearly 16, SS will just drag their heels for a couple of weeks, then wash their hands of it. There are simply too many troubled 15 year olds, and too little money - it makes them very 'pragmatic' and unlikely to help in all but the very, very most serious circumstances. :( I'm afraid they will see DD's BF's mum as an acceptable place for her to be, under the circumstances. I am afraid I think it is unlikely you will get even the (last minute, reluctant) help that Couthy has, because you do not have younger DCs with SN/disabilities, nor a disability yourself. In C's case, they have made the judgement that it will cost more, and be more difficult, if they do not help than if they do. I'm afraid they won't make the same judgement with you. :( :(
I'm sorry to be such a gloom-monger and of course I hope I'm wrong... But I think you have spent such a long time and so much energy trying to get her help, that you need to be careful not to spend the little bit of time and energy you have left on things that probably won't work.
I think your options are different from Couthy'. I think they are:
(a) Ask yourself honestly if DD will be OK at her boyfriend's. It isn't what you'd choose, it isn't ideal... But is it better than having her home? If so, then I'd say you should seriously consider leaving her there.
If his mum refuses, then you are back to the 'choice' you had a year or two ago:
(b) kick her out, or run away yourself, or
(c) hang on in there.
ALL these options are going to be very hard. It is awful - heartbreaking and unfair - that that's what it comes down to.
With ANY of those options, you can continue to ask and fight for help for your daughter. But 16-18 is an awful 'twilight zone' age, where there is very little support.
All of them need you to look after yourself. I seem to remember you saying you didn't feel you could or should look after yourself unless/until you had sorted your DD out... But you are at a point, I think, where you won't be able to help your daughter if you don't help yourself. It's like being in a plane crash: if you don't put the oxygen mask on your own face, you'll be finished before you can help your DD. :(
I am so sorry, Doin. I wish I had a magic wand. All I've got is a ton of sympathy and a glimmer of hope: you have been handling this for years now: you know you ARE strong enough. It will pass. It will get better.