I never even heard of these two as soft markers for downs...hopefully your next ultrasound will go well. I'd try not to worry about it until then especially if your blood results were normal. I think more women get false positives/false scares on the blood tests and if you didn't get a scare on the blood test, I'd try to relax (I know, easier said than done!) and sit tight until your ultrasound!
My son had an echocardiogenic foci (calcium deposit in his heart) which is a soft marker for Down's Syndrome. Like you, I was worried through the pregnancy about it, but I found out a lot of things that made me feel much better. First, the soft markers are different from the markers for Downs. Soft markers are VERY common. One u/s tech even told me that most of the babies she scans has at least one of the soft markers, and the extreme majority do not have Downs.
So basically, soft markers are a load of **** that doctors tell you about just to scare the &*#^ out of you. My son, as you might imagine, was perfectly normal and healthy.
I would not recommend an amnio. As you said, the risk of the amnio is greater than the chance that your baby actually has Downs. Just try to relax.