Michael,

Is there a way with the library you use to get the shell tool to "fill in" the overlaps? Solidworks does this when you run the thicken tool. It creates the inset surface sort of following the end of the non- overlapped bits (e.g. in the example you show, SW would create the inner surface with a sharp edge at the top). SW's thicken command only fails if the thicken distance is more than the maximum space available, so there is no inner surface at all.

Here is a screen shot of what I mean. This is a revolved surface with thicken applied: