@Donald Dahlman I just did something similar. You should choose in advance of starting the project, the type of shower base you want, download the installation instructions (which should be available on the manufacturers website) and familiarize yourself with them. While you are at it get the instructions for the shower door as well, and familiarize yourself with those, too. You may need, for example, to add "nailer" studs so your mounting of the door will go into something solid.
As I recall, the "lip" on the shower base was about 3/16" thick and maybe 1" tall. The one I chose had these little "clips" that snapped onto the lip, and you mounted the clip to the studs. Others may have different schemes. The challenge you may have is that the corner isn't exactly square while the shower base is.
A Hardi-Backer board to use for tiling walls is a nominal .42" thick, just slightly less than typical drywall at .5". If there is sufficient nailer studs, you can butt one to the other and finish them, and then have the finish tile overlap the butt joint.
In the original configuration of my last home, the entire shower area was done first with green board, a sort of water resistant (but not really) drywall, and then the shower area received an
additional layer of tile backer board. So, yes: there was a ½" overlay. The particular tile they used had a liner tile made specifically for this kind of installation. I don't know that many modern tiles come with anything like this.