Today's update. The heater was rebuilt as follows:
New heat exchanger
New pressure switch
New (we didn't have these before) sensors so that if the heat exchanger overheats the system will shut down (in the past the heat exchanger would get so hot the side plastic panels would melt).
Switch that will allow cool down before pump shuts off
As mentioned previously, pool was drained to reduce 45,000ppb phosphate levels, now under 1,000ppb.
Still working on finding a lab to confirm that the scale in the heat exchanger was phosphate.
Tentatively what I think I've learned: Phosphates are probably under-assessed. Often thought of as algae-food, they are potentially a contributor to scale and should be considered if there is, in our case, a heater failure but also SWG issues. Other things need to be ruled out as well including flow and cool-down time.
My plan is to have our man back here in a month or two to check for scale buildup in the heat exchanger.
Thank you, thank you, thank you, to all who contributed thoughts about my situation. I'll post more down the road.
G