The last update on this for a while is that Ben and Karen split for Hawaii before he got a chance to test whether the work they did at the radiator shop cured the problem. Also, he could not get hold of a test kit before he left so even the coolant testing will have to wait a couple weeks for them to return.
I am fairly convinced that the last remaining problem here was bleeding all the air out of the system. Clearly I was unable to do this in the field without the proper tools, or the ability to tilt the bus to whatever weird angle is needed to get those final air pockets out. I am hoping that the process they used at the radiator shop was more successful.
When they get back on March 5 or so we will get a chance to re-test, and I will update the thread here if there is still a problem.