where

In this case, I read it as "in which."

In an extreme case, a case in which a town was...

It seems to me that the relative adverb and the prepositional phrase with the relative pronoun as object (in which) are completely interchangeable here.  But "where" is clearly not a conjunction.

I think "in which" is more common.  I think of "where" in explaining formulae,  "f = ma, where f is the applied force etc."

GG's point was that your "relative adverb where" is not refering to a noun of place as it should, but to a case, which it should not.  "In which" is the correct choice.

You may say, "In a town where a case was pending,".  You may not say, "In a case where a town was abandoned,".  A case is not a place.

I agree with you about the comma.  You're modifying the case on both ends. If you take away the "extreme," the comma goes away.  "In a case where a town was crowded etc."

If you wish to keep the "extreme," I think you'd need to say, "In an extreme case, one where a town was crowded etc."  (Notice that GG repeated "a case" in her example.)

I was leaning toward saying the "where" is incorrect, and must be replaced with "in which," but now I'm not so sure about it. I'm either in over my head or much too tired or both.  Thanks for your patience.

  - A.