seems that every appliance that draws on water from that line could be contaminated with silt. You could do this. they, the plumbers, have told you that the water heater does not have sand in it as well as other appliances. Then with a digital camera, or any camera, take step by step photos of you hooking up a water hose to the water heater and draining it into a big bucket that a picture before hand shows void of any contamination. As the silt comes out of the heater, document this and present to the plumbers and tell them if this has it why would not any other appliance that uses well water not also?
They might get you on the tank as natural sediment naturally occurs there anyway with or without a broken line. But, yo will be able to tell the corrosive sediment from the sand and shell.
As my fellow Moderator would say, document all things said and written. Do not accept a finished job until you are satisfied. If necessary have a lawyer construct a letter of demand and this might let them know that at least you want every water using appliance checked out by them and for you to see each one.