Your other chins need a vet visit ASAP, if the one has a runny nose what you have there is something that can be contageous, chins who get a URI can be dead in hours after showing symptoms. The vet may want to premedicate the existing chins if the second one is diagnosed with a URI. Baytril, ciprofloxacin or chloramphenicol would be big gun antibiotics that would be used by injection, sulfa based antibiotics are not strong enough for this case.
As far as the necropsy goes, the decomposition that would have already occured woud obscure any findings and would be useless.