I can't really answer your questions, and I expect that someone with some actual expertise will be along soon, but will share my experience in case it is reassuring.
My dc1 had 3 soft markers at the 20 week scan - heart, kidney and brain, although can't now remember exact details, think it was also a white spot on heart.
Was told that having the three markers meant a roughly doubled risk of Down's syndrome - no idea if this is based on anything, as my frantic goggling yielded nothing helpful (and I had thought that a soft marker wasn't something that lent itself to statistics). But it was in any case for me a risk doubled from one in hundreds to one in a few less hundreds, so still pretty low risk.
The markers had disappeared by later scans, and DC1 does not have Down's Syndrome. She's now nearly 3, so the details of the scan are fading in my memory - but I do remember it being a very, very stressful time, even though I knew that objectively the risk was still low, so I have every sympathy.
No one could tell me what/why these markers were, just that these things appear sometimes, and they are not anything to worry about in themselves.
Try to remember that the statistics are still very much on your side - I was a couple of years older, with three markers, and still considered low risk. My only real words of wisdom are step away from google, and eat lots of 