Phew! We took another 7.5 hours and finished this up. We followed the advice of the professional and put a scratch coat of mortar on the cement board first, let it set up a bit, then installed the rest of the stones. I still need to patch the drywall in the ceiling where the old fireplace front was attached, clean the stones, clean the slate hearth and build a mantel. And get a fireplace screen so I can remove the cardboard that keeps the cats out. 
