I had planned to make a lot more progress today, but it didn't go as fast as I wanted. I did get up most of the second row, including those two pieces on the left end wall.

Through some kind of miscalculation that I still don't quite understand, I ended up with these two adjacent tiles at different vertical positions.

I noticed it while the mortar was still pretty wet, so I was able to nudge things into better position.

It played some havoc with some of the grout lines. That worried me a bit until I reviewed my test wall. Since the grout is pretty white, it blends in reasonably with the tiles, and I don't think it will attract the eye like a dark grout would.

Update: The next day, Row 3 and the beginning of Row 4. I used a 15 pound batch of thinset and only had a little bit left over. Also, of course, made a huge mess. (Ignore that finger that photobombed me.) 

From this distance, I think the lines look decent. The trim on that top niche looks almost as bad as the bottom niche, so they will both have to have the trim re-done in some way that I can pass off as an aesthetic design choice and not a mistake being fixed.

Update: I had some other things to do on the next day, so I only planned to do a 10 pound batch of thinset. There was a lot of up and down the stairs to shave an eighth of an inch here and there. I had planned to lay the first full-sized tile in row 5, but after I applied all the remaining thinset it still wasn't enough and the tile leaned backwards significantly. You can tell from the photo that I removed that tile and re-purposed thinset to lay another tile on the left end wall.

I've only got two fiddly measure-and-cut operations left, for the upper part of the larger niche. After that, it's just full tiles and single straight cuts.

Update: I did pretty well today on rows 5 and 6. I didn't do the partial tile on row 6 because I ran short of thinset at that point. I also ran out of gas, so I called it a day. Row 7 takes me to the ceiling. It's a vertically short row of about 8 inches or so. In my next session, I can cut the remaining four partial tiles, and then the back wall will be done (modulo re-doing the niche trims), which I'm saving for the end).