My guess is she left it as long as she could to tell you, maybe out of denial, maybe out of fear of your reaction -- she will be showing soon and probably decided she would have to bite the bullet. She will also be feeling the baby move soon, if she hasn't already.
You don't have to have amnio. I had a sonogram, don't remember what they called it, third degree or some such term, to investigate the possibility of Down's for DC5 as I was 37 at the time. The result was accurate. Sonogram took ages and a specialist had to do it. OTOH, amnio isn't half what is going to happen when she gives birth.
WRT the sonographer, the DSD may have heard what she wanted to hear there. And if she heard it later, after the fact, and the mechanics had been sugar coated at the time, everyone would have to deal with that too. It's jarring to have your baby referred to as 'foetal matter' (during one mc my doctor called for the nurse to bring him a sterile container and label it 'products of conception', as I lay there passing large clots in his office) -- medical staff should be a bit more careful wth their terminology imo. But they have a different perspective from the layman perhaps.
I hope your DH will take responsibility to bring the DD to her next appointment. Glad you will be making him take care of the counselling arrangements. Please point out to him that 'not being ready' is the problem, not an excuse. Don't let him off the hook. And I agree, you have every right to expect the DSD to tell you and her father who the father of the baby is -- she owes it to both of you if you are being asked to help her. Give and take and mutual obligations run both ways.
I have an idea that young teenage girls sometimes have inadequate nutrition, try to burn the candle at both ends, and in other ways such as possibly drinking or smoking (not that this is happening here) are under more pressure physically while pregnant than older mothers are. Since a lot of teenage pregnancies are not carefully planned, there is no preparation of the body by taking extra folic acid, and sometimes no prenatal vitamins until late in the pregnancy if it takes a while to secure prenatal care, hence some problems with the babies.