How to tile over shower wall surround flange?
You just want a stable platform for your tile.  It depends on the size of the tile.. If you are cantilevering 10-15% of a larger tile, just fill the thinner areas with more mud and put backer where it will fit. 

It looks like you needed some blocking behind the green board (where the rear curve comes in) to be able to fill in.  Perhaps some fiberglass mesh tape would let you land some thinset to bridge the gap.