Your ideas of a "remedy" are grossly insufficient. Removing the contaminated duct does not cure the problem, its like putting a clean bandage on a cancerous lump. If you fail to address the correct clean up procedure and bring remedy to the cause of the problem - the problem will remain or get worse.
You won't find anyone in their right mind to sub out their labor, you won't find anyone in their right mind to simply remove the 15' of slimy duct.... when someone in your home comes up sick --- you'll be the 1st one to hire an ambulance chaser to sue the pants off the grossly incompetent contractor you hired that should have known better.
Unfortunately this is not a DIY site or a training form, for the most part you have experienced contractors trying their best to offer sage advice.
You have described a serious situation that requires professional remedy.