I know this has been discussed her before with different responses, but
I'm not sure how treat the seam between backer board (cement board) and
green board in a shower area. I realize this is a specific and rather
picky question.

Rather than make the seam exactly on the tile edge, my first theory is
that the tile should slightly overlap onto the greenboard, maybe an
inch or less.

I should use fiberglass tape and I should use the seam side of the
greenboard. In other words, the side that's indented to allow for the
tape.

Questions:

1. Should I try my best to squeeze thinset INTO the seam? I'm not
sure if the argument is to have a good seal or if there would be some
need for expansion between differnent materials.

2. What do I mud with? On the BB side I'll go with the thinset, but
do I also use thinset on the Green board side? Thinset, to my
understanding, dries to a hard fine grain hard cement surface. How am
I supposed to sand that down and get a nice smooth surface that will
accept paint? Is the idea to put drywall joint compond on top of the
dried thinset and then sand that flush? Will the indented portion be
deep enough to allow this? This is my main question and it's been
answered in different ways, so if you're kind enough to offer me some
advice, please back it up with as much justification as you can.

3. If the tile overlaps the greenboard seam slightly and if I'm using
the indented side of the greenboard, then maybe I could intentionally
leave a slightly gap between the tile bottom and the greenboard and
fill that gap with caulking? Or is that just making it harder than it
needs to be?

Thanks for your help!