I think we are both stating basically the same thing. That metal helmets were a defensive response to the new offensive weapon.
Not sure about the comment regarding mostly head wounds with Canister and direct fire Shrapnel rounds. Canister and early Shrapnel rounds produce a cloud of large, .50cal and larger, rounds travelling at quite high velocities and carrying a lot of momentum. It’ll take a hell of a helmet to stop them. I believe that the helmets of World War One were able to deflect and stop smaller pieces of fragmentation only.
Not mostly head wounds with cannister, just with shrapnel. Real shrapnel shells can be stopped by helmets and not just shell fragments. Shrapnel balls are big but they're soft lead and not going at high velocity- even shells with cordite bursting charges often didn't have the velocity to break bones let alone go through or smash helmets. Black powder bursting charges are going to be an even lower velocity.