I don't believe there was any real scientific examination of the question of whether this was needed for safety or not.
You need to put this in context. 1920s context. First there were zero civil aviation regulations until December 1926 when the original Air Commerce Regulations were posted. And 2nd, there were no civil aviation experts to perform any scientific studies back then either.
As touched on in the document I linked above, the main purpose behind the 1926 Air Commerce Act was to foster safety (to include pilot medical requirements) and to regulate aviation across all facets not just air commerce. This started with the Air Mail Act in 1925 and then at the behest of this burgeoning industry, congress passed the 1926 act as the safety record of all flying was horrific when compared to more recent times.
As I recall, the only military connection with the process was one could receive credit from any military medical exam or training when applying for a civil pilots license and medical exam. There was no pilot medical license or certificate at that time as it was part of obtaining your pilot license. Same military experince applied to the mechanic side. And it was a license back then vs a certificate as today to include each aircraft was labeled as licensed or unlicensed.
A review of those original ACRs from 1926 will show you how things were then. Unfortunately, with the recent upgrade to the FAA DRS, all the links to the DOT library historical pages are gone. Best bet is to search the National Archives for those if interested. But keep in mind the scenarios and rules back then were very different than today and those rules changed on a regular basis. For example, the ACRs were revised almost monthly and by 1928 the Aeronautic Brach had expanded to include an enforcement division and various other divisions.
Regardless, when the legendary Knute Rockne was killed in an aircraft accident in 1931 the path was laid to the reorganization of the Aeronatics Branch, which led to the creation of the CAA/CARs and so on. There's a lot of historical info on this if interested