Final update:
CasaInc sent me new check valves (the small circles that the water was coming out of in the images and videos above). I was expecting to receive a completely new valve body. However, I removed the existing ones, whcih i hadnt done yet, and wouldn't you know it a little bit of wood shavings plugged in that hot water side. I flushed it out, reinserted the check valves... and it's working! I actually did consider this, but didn't find the culprit until now:
So in the end, user error... although perhaps not all valves clog that easily, I don't know. Feels a little silly to have gone through all this and in the end sawdust was the problem... but glad to have it working. Thanks all for your help!
CasaInc sent me new check valves (the small circles that the water was coming out of in the images and videos above). I was expecting to receive a completely new valve body. However, I removed the existing ones, whcih i hadnt done yet, and wouldn't you know it a little bit of wood shavings plugged in that hot water side. I flushed it out, reinserted the check valves... and it's working! I actually did consider this, but didn't find the culprit until now:
The cartridge being jammed with something seems possible. The valve came assembled and I have not tried to take it apart yet. All I can think of is that once I had it in place I did drill one more hole in the stud above so there was some sawdust that landed on it and maybe some got inside... we'd be talking falling a few feet into an opening the size of a 1/2" pex fitting, but maybe that's all it takes! In that case it would have come in from the shower outlet opening.
So in the end, user error... although perhaps not all valves clog that easily, I don't know. Feels a little silly to have gone through all this and in the end sawdust was the problem... but glad to have it working. Thanks all for your help!