The layer is called the Periostracum and is composed largely of protein structures. I have never actually considered the snail shell to be a "permeable membrane" but maybe since this outside layer was able to deteriorate due to some issue, maybe it can regenerate? I will see if I can find a source which explains if this layer is being constantly secreted through the shell
Once this layer of shell is gone, it won't grow back, it is secreted by the mantle when the shell is first laid down. The snail can heal shell damage from the inside, anywhere on the shell, but not replace this layer.
It won't harm them to lose the top layer, except they may be more suceptable to acid damage
Dead snails lose this layer of shell because they are laying out in the open and exposed to the elements; it wears away over time, older snails will have significantly more wear especially wild snails living in a rougher environment.