It is natural to be concerned after learning results from a screening test that show an increased risk for a chromosomal problem such as Down syndrome. You are correct in that the odds are in your favor for the pregnancy not to be affected with Down syndrome.
As you know, first trimester screening is an early screening option for a few of the most common chromosome abnormalities seen at birth including Down syndrome. The screening involves a blood test and an early ultrasound (about 9.5-13.9 weeks). The results of this screening test can help some women decide if they would like to go on to further diagnostic testing such as CVS or amniocentesis. An amniocentesis can provide a yes or no answer to whether your pregnancy has chromosome problem such as Down syndrome. Amniocentesis has a risk for miscarriage that ranges from 1 in 200 to 1 in 500.
Looking at the HCG levels in your urine would not help provide information about the chances that the pregnancy is affected with Down syndrome.
Regarding your carrier tests for Cystic fibrosis (CF) and Canavan disease, these are not related to your first trimester screening results. Since you are a carrier of CF and Canavan and your husband is not, your chance to have a child with CF or Canavan disease is greatly lowered but not to zero.
Although first-trimester screening results may vary in subsequent pregnancies, the results from your carrier screening for Canavan and CF do not change over time because they are DNA based.
Prenatal testing for birth defects is complex and quite personal. Women have many options and should think all of them through considering the medical facts and their own personal/family values. I am not sure if you are scheduled to meet with a genetic counselor before your amniocentesis, but you may benefit from discussing your options with a genetic counselor. You can locate a genetic counselor through the NSGC website. Best wishes to you.