While in the CWM recovery menu I realized that I actually like the stock ROM and really didn't want to invite "experimental issues", so I went for a wipe and factory restore. When it rebooted after and I went through setup, I made sure to uncheck the Google restore options. I even removed the SD, copied all backup and recovery directories to the computer and deleted them from the SD.
Every single bit (no pun intended) of everything was reset to factory.
Installed Moon+ Pro. Started it up once. Installed CC. Started it up, made preferred changes to formats / folder locations. Connected to CS and downloaded some books in all three of my formats - epub, pdf, mobi. (I should note that nothing in calibre has DRM, so that's never an issue.)
Selected a PDF - opened in Moon+
Selected a mobi - opened in Moon+
Selected an epub - Moon+ did not pick up the call.
So I open Astro file manager, and just as before, everything including epubs open in Moon+ just fine from there. I even forced it to set Moon+ as the default for epub but still no joy in CC. Specifying a MIME type doesn't do anything either. (That was a long shot anyway - mobi doesn't have one set and they work just fine.)
At this point I'm ready to give up on having CC be my main interface for browsing my books. I love it but there's obviously a problem specific to my tablet that I can't get around without more drastic measures. My earlier experiments showed that using a different reader app doesn't fix the epub issue either.
Thanks to the three of you for your help. At some point I may take another swing at it but right now I'm moving on to the other fun things my tablet does.
CC is still very useful to me even without this.