Lastly, we've never established any "qualification" for an "investigator": everyone shows up, with their own perspective and background. Often the paid "investigator" is the least qualified of the group (workers from the industry).
This is difficult to understand
Who is the "we" in the above
What's a "paid" investigator vs an "unpaid" investigator
If you refer to ICAO annex 13 for guidance than "expertise" is the only guidance that I can think of and even there it does not distinguish what kind of expertise. Mostly what is available are a collection of enough expertise to get the job done and in that the most critical expertise are those that know what they do know vs what they don't know.
Recognizing what you don't know and and then seeking help as needed is what gets the job done. If by industry folks you mean the manufacturers then they often serve as conduits via telephone to sources of specific expertise rather than as all around investigators themselves.