Thanks for the help Rico. As a reward for your assistance, I pile on a
few more questions....

Regarding not needing a "big" gap between the wall and the cement
board, I was hoping for a more concrete answer (no pun intended). I
know with plywood you have have to allow for expansion. Is that the
same thinking here with the cement board?

Regarding caulking the wall-to-backer board gaps, is it best to use
100% Silicone or is a tub/tile silicone better? (To be clear, I use
thinset on the backerboard-to-backerboard gaps).

Finally, in true amature form, I don't know the difference between
modified and non-modified thinset, but from what I've read I'm to use
non-modified thinset to bond the backerboard to the plywood. Another
source said to just use constructive adhesive. Any thoughts on that?

Thanks again for any help you can offer!