Looking for ideas for a shower back wall / tile size problem

Then just use some mosaic in the back wall to get the offset between light and dark tiles.

You could also paint the niche white again ... or actually black with mosaic in between, etc. Then the niche is not exactly one tile high, but 1/4 or 3/4 .. then some metallic mosaic and then a bit of tile.

Then put the black tiles vertically and start at the bottom with a 3/4 high tile or so.

Have you measured the tiles yourself or is the measurement only on the box?

If the niche causes so many problems, close it and tile over it