Leave. What would the leave arrangements for American troops have been like? I know a lot of British soldiers went back to Britain on leave, would the Americans as well? Or would they have their leave behind the lines still in France? How long would their leave be? Who would command a company if the captain was on leave? Would the captain of a company and the lieutenant of a platoon likely have leave at the same time?
Injuries. One of my characters (the captain) is shot in the shoulder. I want it to be a fairly minor injury, no muscle or bone damage, no infection. What would his treatment be and how long would he be out before rejoining his company? And again, in his absence who would command it? I imagine in this case, with it being longer-term than leave, another officer might be posted to the command perhaps?
Prisoners-of-War. How would POWs have been treated by the Germans? Were they shipped back to Germany, or kept at camps to do labour? If they'd been injured, would they have received medical care?
I've tried every permutation of AEF and/or doughboys + western front + WW1 or World War One I can think of. I've tried Wikipedia, I've tried JSTOR and my university's journal archives, I've looked at some of the memoirs and books on Project Gutenberg, I've looked at the resources on the official 1st Division Museum site, I've requested what books I can from the British Library.