i would never use a premix grout!!!!!! i add a Laticrete additive to powdered grout that adds a sealer and extender to the grout.
for adhesive, i add the same additive to thinset for floors and use the best mastic for walls, even in most showers, without callbacks......i still have the same customers and still doing work in their homes.
their have been some situations where thinset works better for custom showers, especially if they are 2 person and have very expensive tile and tile details.
i use either 1/2" hardiboard or durorock for the shower walls.
good luck with your project!