I'm not aware of any studies that have specifically set out to determine whether the incidence is different in a same-age, same-circumstance elderly male population who aren't veterans versus those who are. I'm not aware of any studies that have taken that actual approach.
Based on what we do know about Alzheimer's, I think your second point, that there really is more in the genetic and the environmental area, would explain the incidence. I don't think there have been any studies that have specifically addressed whether trauma-based combat situations contribute to a higher incidence of Alzheimer's. I would be surprised to find that the incidence was different in that population, but I don't think the research is being done.