Hi Judy in Kentucky.
 I don't know what the USDA zones mean but I doubt if they are relevant to
your problems.
I am sorry to read of your losses. They must be distressing as nobody likes to
lose their bees especially such a large proportion.  I haven't got an answer
for you, only more questions.  If you lived a little nearer you could join our
local association's bee buddy system and somebody would come and lean over
your shoulder to help and advise.  If you are a member of your local
beekeeping association perhaps they could help.
From your mail you seem to have covered most of the likely possibilities
although I have my reservations expressed before on the list about treating
your bees for problems they don't have.  If nothing worse, the treatment
(putting something in your hives that the bees wouldn't themselves bring in)
must at least be stressful.
Your bees were clear of mites.  Did the lab check for nosema (unlikely to be a
problem in summer) or viruses (more likely), or poison sprays?  I assume you
can recognise AFB and EFB and that they were clear.
The problem must be fairly local and specific to your bees or you would not
have found swarms in the area.  Have you treated your hives with preservative?
Have you left them short of stores?  Is there a nearby supply of water? Are
they in a place where they are exposed to excessive wind and weather or
disturbance from animals?  What crops are being grown locally and how are they
treated?  Are you downwind from a nuclear power station or military
establishment?
I probably haven't asked the right question but I hope I have given you a clue
as to the sort of question you should ask yourself.
Chris Slade (temperate maritime climate, don't ask about the weather) about
51N 3W