I understand what you are going through.I, too, have been trying to reconstruct my dad's service in Marine Aviation.If you have the records from St. Louis, then the next thing is to get any unit records you can from the National Archives II in College Park, MD.All militaryrecords over 50 yrs old are there.They have Specialists who can guide you to the right records.A Mr. Zubrin or Mr. Smith are the tops in Marine research.You can also call the Washington Navy Yard and talk to the USMC Historian, Mr. Aquila.He might have some clues for you to follow.
According to my research for VMF-214 aka Black Sheep Squadron (but long after Boyington),the training went on at the various sites in CA.Then when a unit was put on a carrier or deployed in the field, they left a rear echelon of replacement pilots, mechanics, ordinance specialists, and so forth.I don't understand why they took all the planes, quite yet.The guys in the rear had to continue training on something.
Back to my point.I got the Muster Rolls for the unit during the time my dad was in it. That will list his assignment and where he was every day of the month. Then, by miracle I was able to find some of the other enlisted men who were still alive.I called them asked them about what they did and where they were at various times.It gave me a pretty good idea about what circumstances helped my dad survive the bombing of the USS Franklin (CV-13).To follow up, I wrote each one a thank you and sent a picture of my dad just in case it jogged some memory somewhere.
I agree the records of the stateside Marines or the training groups is sketchy at best.War Diaries are much better but not informative about the enlisted men.I dare say the war left many men with psychological scars.The whole creed of "real men don't show weaknesses and don't talk about it" took a toll we will never be able to measure.
If you have not written to St. Louis, do that.Something might be in there that is not in what you have.I found out that the USS Marshall picked him out of the sea that day.Never knew that before.Got the deck log for the Marshall at NARA II and it had the list of men it recovered.Found 3 of the other Black Sheep who'd been picked up with him still living and called them.
There may have been a training accident or something on the civilian side or the death of a close friend, or a close call, which probably won't show up in records.I am luckier than some, in that my dad was on Guadalcanal and on the Franklin, which are heavily documented (but the War Diary for the VMF 214 were destroyed in the fire on the carrier).So even if he did not tell me what and why he was changed (he would absolutely never ever fly, period.We all flew where ever and every where but under no circumstances would he even consider it.Based on his war time experiences he was sure if he did, he would die.), I can get an idea of what the environment he was living in was like.
Maybe try a local news paper or some logs for the various bases.The Deck Logs record the Courts Marshall that were held, punishments, etc.Surely bases have the same reports for soldiers who violated some regulation.If you really want to know, you have to cover every angle.