Suggestions for finishing off bathtub ledge
Take a piece of corner tile and seal it to the top of your ledge so it overhangs your removable cement board. See picture below.



They come in all sorts of lengths. Tile your removable cement board and fit it  up under the corner overhang.  Think about taking a 2" strip of cement board and screwing it to the framing in contact with the floor and wall so you could caulk it to the floor and wall to prevent water from getting in there. Your removable piece would fit on top of the strip and under the corner piece.