Jim I just went through your entire thread and I don't see anything that would cause any movement. The blocking doesn't appear that it would or could be the cause. I would be curious to know what is on the other side of that five foot wall.
I did notice the studs used for the shower appear to be 2X2 but there were so many great pictures I didn't read the whole thread to know if that is the case. Obviously the studs are tightly shimmed to whatever that wall is behind them.
My first "guess" and it is only a guess is that the cracking could be caused from some kind of impact to the wall. Hopefully no one has fallen while in the tub/shower.
I'm wondering if the issue that caused the cracking could have come from the other side of that back (5') wall.
If unsanded grout was used (and it appears it was) that stuff has been known to crack on its own. Especially if mastic is used for the adhesive.