Amnio results are real and will not give false positives, at least for Down's. The DNA either has an extra 21st chromosome or it doesn't.
I had amnio for my first two pregnancies and I was in my 20's. For the first. my son had some soft markers for an increased chance of Trisomy 18. For my second, his triple-screen test (which is now the quad-screen) came back with an increased risk for Down's syndrome. I would not have terminated in either case, but I would have changed my plans for birth, maternity leave etc. if either amnio had come back positive. Thankfully, both were fine.
Amnio isn't just about terminating pregnancies, it's about knowing whether or not you have to prepare for a different birth experience or will be caring for a newborn who has health and development problems. For example, with Down's syndrome, one might want to deliver at a hospital that can provide a higher level of neo-natal care and has surgeons on-site who are prepared to do surgery on the heart or spine soon after birth if necessary. If you work, you would need to line up a childcare provider who is experienced with and capable of caring for a child with health and development problems. For something like Trisomy 18, one would probably have a c-section to lessen the physical trauma that vaginal birth would cause a T-18 baby, etc. and prepare oneself emotionally for the possibility of still birth or early infancy loss.
For me, the amnios made sense because my own doctor's record of miscarriage from amnio is much lower than 1 in 250, the odds of these very challenging problems were relatively high (1 in 200), and I didn't want to have the rest of my pregnancy overshadowed with doubt and anxiety. I wanted to know, either way, right then and there.
If you're unsure of what to do but know that termination isn't an option, then you have the option of more time. Perhaps you can wait until a later u/s gives your a clearer picture of what you may be dealing with, if anything. There are many soft markers for Down's - they're not as accurate as an amnio, but things like the presence of the nasal bone, the length of the femur, the absence of a white spot on the heart and other physical indicators can be good signs that there is nothing to be concerned about. Maybe you can have another u/s weeks from now that will be more reassuring instead of amnio? Or if you opt for amnio, maybe it's possible to do it later when if you were to go into labor, the baby would have a chance of survival? I don't know if there is a window of time when amnio is not possible.
Anyway...I'm sending you big hugs. This kind of thing can be so scary - I wish you the best, whatever you decide and whatever the results say.