How do I handle expansion joints in corners when tiling a shower?
I was told a decade ago when doing the same thing to not worry about expansion joints in the panels but instead do them where the corners are as the expansions come from shifts in the walls in relation to the floor - basically anywhere that the tile isn't flat but makes a change in plane.  In those areas use white silicone caulk not grout.  Seems to have worked fine.
As for expansion joints between the cement board and the wall itself - didn't put anything like that in.  Only put plastic in between the board and the wall.  I'm not sure you would want the cement board floating against the wall.