IMO, Dan, if you use Kerdi band only on the end walls you present an opportunity for water from the back wall to get in behind the band - unless you are able to completely seal the ends of the band.
- I would install the band first, then RG over the edge of it.
- I would fill the holes with mortar before RG'g them.
- If that niche is made of a water proof material I see no benefit of applying RG to the inside of it.