This fear mongering pops up every few years. 1918 is never going to happen unless we go back to millions crammed together on troop ships, houses with no indoor plumbing, and most being ignorant of basic sanitation. That’s why things like Ebola strike backwards regions but quickly die out in the 1st World.
Wrong. The flu of 1915 was worldwide, not military connected. It was a virus that caused quick deaths to extremely health young people because their immune system overreacted and they basically suffocated. Death within 24 hours was common. It seemed the healthier you were the more likely you would die because the immune response was stronger.
Plumbing has little effect on the flu.
As for the rest. Have you been to a major city lately?