Hello Tess, I've been reading this thread since the start, although on the second one, I've been skimming it and focusing on your posts. I really feel for you; what a difficult situation.
I think you're handling it brilliantly.
I wanted to address the issue of the nuchal scan/amniocentesis. From what you said in your post, someone has posted that teenage mothers are more at risk of having babies with SN, therefore people are advising you to push DSD into having an amnio. The nuchal scan, as you know, is a screening tool for Trisomy 21 (DS), whereas amniocentesis is a diagnostis test for DS, and also Trisomies 18 & 13 (Edward's and Patau's syndromes). Teenagers are at very low risk of having a baby with any of the Trisomy syndromes; the risk of these increases with age. To be honest, therefore, I wouldn't think having an amniocentesis would be particularly useful. I've seen it done, and it's not a pleasant procedure.
There are, however, other congenital malformations which are more likely in the babies of teenage mothers. Here's a paper about that, and I'm sure you'll find lots more if you search on pubmed (the site I've linked to.
humrep.oxfordjournals.org/content/22/6/1730.full
The anomaly scan, which is done at around 20 weeks, is the one where they scan in great detail, looking for abnormalities. If there's any problem, they're very likely to pick it up then.