I'm really glad you liked this so much. I love the idea of the Trafalmadorians as an artefact of PTSD. It pre-dates the official recognition of PTSD by a decade but I think you're right: Vonnegut knew exactly what it was and what it did.
The way he travels seemed a lot like PTSD flashbacks to me, and there are clear associations between Billi's later life and the scenes recalled. I think Vonnegut knew exactly what he was evoking, even if the term was not the modern one then, or widespread.