When was the "retrospectoscope" first used?

Solution 1:

The earliest example I found is from 1940:

Instead of being listed as histories, prophecies and wills of the class, were on the program as "retrospectroscope," "futurama-scope" and "etho-cosmascope."
_{The Emporia Gazette from Emporia, Kansas: January 26, 1940 (newspapers.com subscribers only)}

Given how early this is compared to the other examples, in addition to the fact that it has a slightly different meaning than the sense you mention in the question, I'm not sure it's related to the term used in the medical field or not.

The next most recent examples I found were from 1954:

As Dr. Merrill Sosman of Boston has often pointed out the "retrospectoscope" is a wonderful diagnostic aid.
_{The association of gallstones and heart disease}

Adm. Lewis L. Strauss, chairman of the atomic energy commission used his "retrospectoscope" the other day to recall an experience with the late Sen. Smith W. (Wild man) Brookhart of Iowa.
_{Corpus Christi Times: May 5, 1954; The Independent Record from Helena, Montana: May 9, 1954 (newspapers.com subscribers only)}

As for usage, it seems to be mostly confined to the field of medicine, with some usage beyond that. There are quite a few hits for the word in the title of scholarly papers (in the field of medicine of course), like this one. Some of the results I found, like this one and this one don't seem connected to medicine.

Solution 2:

this ngram sheds light on usage:

I'd like to know when the word retrospectoscope was first used?

The Emporia Gazette cited elsewhere is one of the earliest uses.

Has use passed into the understanding if not usage of non-medical people today?

Yes. It appears to me and by my research, the following links show diverse and continued use this year. (U.S. articles):

black history month, episcopal blog, a reply in a U.S. tweet, a jogging blog, medical article