Tess - have been watching this thread and just wanted to concur with all the people who have praised you for being so strong, caring and above all bloody reasonable in the thick of an impossible situation.
I agree with the poster who said that, harsh as it is, your DSD can no longer play the 'child' card now that she is pg. I think the more time you and H spend arguing about who will be 'raising' the baby, the more you are sending out signals that it will be someone other than DSD. Yes of course you must all support her (and you and H will have to decide how you divide any workload as grandparents) but ultimately if DSD is having a baby, she must raise it. That means she does all night feeds herself and can't use her 'age' as an excuse for copping out. In reality, once you have a baby, as we all know, you can't 'skip feeds' because you are tired/studying/hungover/have a cold. Many mothers of all ages study and work whilst being a parent, we don't expect someone to do the night feeds just because we have to work late, and just because you are teenage mother and have someone as kind as you in the same house, you can't be led to beleive that it is a 'shared' responsibility - it isn't, not really.
I also agree with all the posters who have encouraged your DSD to get a better understading of the day to day responsibilities of beimotherhood - it does sound as though she needs a reality check, not necessarily in order that she rethink the abortion (although this may happen) but because she sounds very naive and she has a big awakening ahead of her. She may turn out to be a fantastic, young mother and I hope she does, but she does need to accept (as does your H) that she will be a 'mother' in practice as well as name. You, Tess should only be there for emergencies, encouragement and support - not day to day childcare.
Good luck with the scan and with getting your H (who sounds as though he is being an arse because he is shell-shocked and a bit lost) to support you better throughout this.