I know the US lining in their WW2 vintage helmets was removable for easy washing/cooking/digging/bailing water. I also know that our modern military has adopted a stahlhelm type design which better covers the neck and sides of the head.

The sad truth would be that no matter design or material you used the helmet was made for deflections or indirect shrapnel. Any helmet from that time would not stop a rifle round dead on.

The best helmet would probably depend on your role in the military and your location. A rifleman spends a lot of time on the ground, possibly being shelled and so neck coverage and back of the head coverage are important. An air force crew member has different dangers and probably would benefit from different helmet design.

Ex NCO here, during my time at the Military Academy similar questions came up. In general, and based on our tests, the German model was the preferred one. We had available German, Italian, American and Russian helmets (no Brit nor Japanese but honestly nobody like them anyway here), overall the German came first (our army used a similar one for many years) and the American would come second. Both Russian and Italian were similar to the American in shape but in general less comfortable. Despite the protection fact, limited in all of them to protect you from splinters, indirect lose bullets, some shrapnel and debris, none would stand a direct shot under certain range (sorry we did not shoot them to try the range, commanding officer would not be pleased). I have to say that the German model gave a sense of better protection covering more surface, particularly around the back of the neck with its flap. This is a small functional advantage and somehow a psychological one. All other helmets resembled pretty much those used by construction workers and they felt more "open". Worth mentioning that I was at the Academy back in the late eighties which coincidentally was the time at which the US Army adopted a similar helmet design to that of the WWII Germany with its back neck flap.