Hey Joe,
The Pickelhaube was not completely replaced by the Stahlhelm, The two M1915s that I have were made in 1917, and 1915 with a BA VIII stamp dated 1918. as far as the neck flaps go on the Mütze, It looks like that same flap shown in the Prictor collection. I think that the benefit of covering the neck and ears would be very appearent to a fairskinned soldier very quickly after arriving in a tropical region, and I would not be suprised to see photos of soldiers with kerchiefs tucked under their caps/helmets.
Gus