what should we do at the chipped tub rim lip
Option two is the way to go. You don't want any angle space between the studs and the cement board. Secure the cement board firmly to the studs flush with the edge of the tub lip and seal the seam with caulk. Then install your tile over the cement board and face of the tub lip. Then grout and then seal the bottom row of tile where it meets the tub with caulk.