I think a soldier is classified as a veteran the day after his discharge. In world war I the term shell-shock was used to describe the condition World War II used the term battle fatigue and now it's PTSD. The soldier may well have returned home and lived with his family before it was decided he was suffering from severe shell-shock and sent to the institution.
The files lost in the fire at St Louis were the collection that was arranged by the name of the soldier. They can work to reconstruct the information from the name of the soldier and the details of the unit he served - that is - the division number, type of unit - infantry, engineers, artillery, machine gun, etc. The reconstruction is based upon the documents called Special Orders which each unit issued that detail the name of each soldier when he was assigned to some particular duty. There can be special order documents for each day of the year and some of them will have over 100 names so you can imagine what a time consuming task it is to find the paper for a particular name.
The US National Archives has records for the WWI divisions at their location in College Park, MD. At their web site select Search ARC - Archival Research Cataloghttp://arcweb.archives.gov/arc/action/BasicSearchForm?jScrip
put 301641 in the search box - this is the ID number for the catalog description of "Records of Divisions". When you locate the description click on the tab "Archived Copies", then click on the "View Container List" which lists SOME specific boxes for each division. Note this box list is INCOMPLETE. There are twice as many boxes listed on the paper finding aid in the consultation room.