Agree with Spidookly - that's a powerful argument that DH needs to hear. He has to man up so that DSD sees that he is there for her, and show her that, unlike her mother, she does have a very concerned parent, here, now, and who loves and wants the best for her, here and now. This comes full circle to the root of the problem: DSDs feelings of rejection at the hands of her mother and her desire for a baby to repair that damage.
I think DH needs to reassure DSD of his love for her as well as step up to the plate practically: Words AND actions (attending appointments, doing research, organising counselling). He needs to tell her how glad he is she came to live with you and how lucky he is because a lot of separated fathers don't get that opportunity and he has, and also that, while he is angry with her for putting everyone in this situation*, he still loves her and always will. It is her mum's loss, and even if he hadn't wanted it to happen the way it did, it is absolutely his gain. Even now. And that he is going to organise counselling so that all these issues can be spoken about.
That is absolutely no criticism of you Tess (and Tess you are very much her parent as has been shown by all your posts) and the love you have shown your DSD. It is just pure biological fact, in the same way that adoptive parents I am sure realise why their child's desire to find their biological parents is in no way a rejection of them or their parenting or love (although I should imagine still exceptionally tough for them to deal with emotionally even if you've always been preparing for it mentally).
And Tess - do go away. Plan something fun with the boys if you can - is there someone you can go and stay with and do Halloween things with for a long weekend? Ask DSD and DH to sit down together and talk about how they are going to practically approach this, and that will not include you caring for the baby anymore than a GM who lived 30 minutes away and would pop in for visits.
*Having followed the thread I do realise that it is more than likely this was a planned pregnancy, but that doesn't make me feel any less sorry for DSD because as a 14 year old she has acted on deep-seated desires without thinking of practicalities - which is pretty desperate.