Re: Crown Molding - why hollow behind?
Crown is "hollow" so that it will bridge irregularities in the wall, also so that it requires less material and is less weight.
The easiest way to handle the crown at the shower will be to return it back to the wall. That is, instead of "dead ending" or butting it into the shower, you cut an outside corner on it and then a mating piece that butts it into the wall just short of the shower. You could also use a corner block at the end, but that will probably look a bit strange.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!