Most instructional videos and articles I've come across favor the back butter approach, and a 3/16 or 1/4 notched trowel on the wall. Too little mortar and tiles can pop off easily. My tile substrate maker (Kerdi) specifically recommends a 3/16 x 1/4 v-notch trowel in some parts of their literature, but they have several application videos on Youtube where their people have additional recommendations that pertain more to tile coverage and adherence percentages, and to increase trowel size to get better coverage and adherence. But back-buttering is important to ensure that even off-plane tiles with a little bit of bow or arch to them adhere fully to their substrate. I've got 3 different trowel sizes and if for some reason they don't seem right when I start in on the tile, I'll go out and get one more.
Basically, thicker mortar allows for uneven-ness in your wall to not show through the laid tile. I have places where my wall anchors for my kerdi board bow it inwards a bit, and I'll have to use thicker mortar there than other places, but that gets averaged out when applying mortar to the wall in the first place. There just has to be enough, without being too much.
As for mortar pouring out between tile, that's what a margin trowel and a wet sponge is for.
Was loosely intending on sanded grout for the floor. Floor is a riverstone pattern. Walls are going to be 12x24 porcelain. Haven't really thought about grout for the walls yet, other than I favor smaller grout lines for the porcelain.
I do have a proprietary Schluter caulk/sealant intended for use where the tile meets the floor, and the corner seams of the tile. That will certainly be used to close all the seams of the "box" of the shower, rather than grout in those channels. I could probably just use that same sealant for all the seams of the porcelain tile rather than grout. I don't really know what it looks like so I may hold off on that decision until I've tile the walls and applied the sealant to the box seams.
Grout really hasn't hit my research radar much yet.