I reviewed the 6175,0 article several times before posting this inquiry and tried the steps outlined.  File Doctor would run and come back with 'No problems found'.  I could open each company as single user/not hosted from the server (local QB 2018 is installed), ran backup with verification and database integrity check. These all worked.  (and why would I need a third party database repair utility? Can't Intuit fix their own database??).

After trying the same steps several times (definition of insanity?) and having File Doctor come back and tell me 'turn on hosting' again, I went back and tried it.  Suddenly it worked-all companies now showed multi-user hosting.  I have no idea what/if I did differently, but now it is OK.  This happened a couple months ago and I figured it was a fluke. Apparently not; I just wish I could figure out why QB decides to have a hissy fit and stop hosting.