Glasses can be worn with the goggles helmet, they will be visible and in front of the goggles.
The DT even at max condition is quite low. This is due to the fact that steel helmets were not meant to be bulletproof, rather protect from flying shrapnel and debris. The helmets would be quite useful while inside old buildings, as they would protect from falling chunks of building material, and the brim would be useful outside, to protect from the sun.
According to John Sawyer, the helmets "likely would have used metal with a cloth over the top, which was fairly common in the 20th century." They are most likely made en masse out of melted down scrap metal and pressed into shape.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
The helmet appears to be influenced by both the Brodie helmet, as used by most Americans and British commonwealths in World War I, and wide-brimmed pith helmets, used before, during, and after the same time period.
End of information based on unverified behind the scenes information.