Imagining the particular thoughts of particular individuals is not something historians can do and still be regarded as historians.
But imagining the possible typical thoughts of typical types of people is something historians do all the time --- and must do all the time.
Science knows of many similar situations in its own line of work ---- for the scientists can never predict the behavior of a single gas molecule or when exactly a particular radioactive atom will split, but they can predict the typical behavior of trillions of gas molecules or trillions of radioactive atoms.
Pollsters do somewhat the same at the other end of the scale --- interviewing a few thousand carefully representative individuals for their particular opinions will let them predict the probable behavior of millions of others rather like them.
If we read of the reactions of many hundreds of typical doctors, patients, familes, administrators to a new diagnosis of terminal SBE disease, we can successfully imagine what a typical doctor might say to a typical new patient, if not what Dr Henry Dawson did say to this particular new patient on this or that particular day.
Historians gain such insights by simply sitting down and doing the bull-hard work of plowing through hundreds of books and articles and thousands of pages till they develop a feel for the consensus on how America generally viewed SBE in the early 1940s, for example.
It forces a historian to become a narrow specialist because they lack the relatively ready and easy access to current reality that the journalist automatically enjoys.
I can't imagine in writing another book beyond this one on the story of wartime penicillin - for in my own way - I am a perfectionist and can't imagine ever having quite enough facts to feel I can truly tell the tale accurately and completely.
It must always be a work in constant progress...