PTSD he might have suffered from surviving a very violent and almost certainly fatal accident
PTSD is caused more by sustained high stress over long periods of time, not by single incidents. Construction workers get into accidents all the time, if sudden enormous changes in behavior were common due to singular incidents of accidents it would be a well known and commonly heard of phenomenon. Let's see some evidence of that happening elsewhere.
If you aren't willing to accept the Phineas Gage research due to just making up some unfounded objection to every element of evidence then no shit you'll never be able to know anything about how material conditions might affect behavior. Of course if you a priori rule out the possibility of knowledge you have to conclude no knowledge is possible lol.
Just because the knowledge isn't perfect doesn't mean we should throw out the whole field of study. Human behavior and psychology is really obviously affected by natural selection otherwise every baby would just behave totally randomly with no pattern of development. Human behavior would all be totally random and unpredictable at all times.
Two siblings in the same family with the same upbringing can turn out wildly different
Sure... But
generally speaking people with similar genetic background, look, sound, smell, achieve, behave etc similarly. The bigger picture you go the more predictive these generalities become. I just don't know how someone can accept that ones genetics determine hair color, skin color, muscle distribution, height, weight, face shape, every single physical aspect... But the brain is a mystery, untouched, inscrutable. The one organ that uses more resources than any other organ proportionally by mass is magically immune to selective pressures. All people at all times just randomly turn out with a totally unpredictable set of psychology traits. The only way someone can think that is consuming massive amounts of globohomo propaganda, which, to be fair, is the typical upbringing.