2. if tile goes only up to a certain height on the walls, i.e. not all the way up to the ceiling, and assuming that it is not possible (or feasible) to be aligning cement board level exactly with that height, should the cement board go slightly OVER or slightly BELOW the tile line ? in other words, should the tile top line be immediately running over cement board or green rock ?
1) Never hurts to use the green stuff. I'm not convinced it will make a difference, but with all the humidity in a bath, why not?
2) Hide the transition between the wonderboard and the sheetrock just below the top of the uppermost tile. That way you're not pulling hair out trying to hide that joint. Just keep the joint an inch or two below the top. Tape the joint and "mud" it using mesh tape and thinset.