PTSD

Is it even possible for World War One to have had more ground for fostering PTSD?

Well, maybe, but trench warfare and PTSD certainly go hand in hand. Any type of warfare is enoguh to give someone PTSD, what with the constant fear of being shot at or bombed and dying, but trench warfare was really extreme. Poison gas was a horrible way to die, and watching someone die by it was almost as horrible as you being the one injured. You try watching one of your comrades drown in their own frothy blood, and not be haunted by that image for a very long time. And unlike traditional warfare where the dead were buried after each battle, trench warfare had no letup. If your friend died while fighting right next to you in the trenches, his body stayed there. At least until the rats got to him. Even the 1918 influenza outbreak would contribute to PTSD, with the dead so numerous that they were stacked up outside the hospitals, and there was a shortage of coffins in America.
It would be very difficult to argue that World War One resulted in no cases of PTSD. It would probably be difficult even to argue that cases of PTSD were few, even if they weren't diagnosed. With the conditions of trench warfare, severe trauma almost seems a natural after-effect.

Sorry if this was a little too graphic.