Sorry you're going through this. I can understand the anxiety you're experiencing. You want to know exactly what's going on with your baby, whether you choose to do anything with that info or not. Knowing will just help you sleep at night.
An echogenic focus in the heart IS a soft marker for Downs...but it's a soft marker for a reason. Many many many normal babies have these calcium deposits in their heart and they DO fade away. Having it there will not affect how the heart performs or grows. If no other markers (soft or hard) were seen and your bloodwork screening tests came back within normal limits then chances are your baby is just fine. The reason they recommend an amnio is because they're saying your risk of having a Down's baby is greater than having a miscarriage from an amnio. That's also why they recommend getting an amnio once you're 35.
But it's a very very personal decision. No one can tell you what to do nor should they offer their opinion. You, not them, will have the sleepless nights not knowing OR the miser if something were to go wrong with an amnio. It's a decision you alone have to make. It was only one soft marker though, and with normal bloodwork you're baby is most likely perfectly fine!! Just do what's in your gut...either way.
Best of luck!