More good/bad/indifferent news...
Good news: I redid the heater hose plumbing, replacing the heater bypass valves that I was using, with heater shutoff valves that are specified for a late model Chrysler K-car. I tested them before installation, and they do appear to completely shut off coolant flow when they close. So, perhaps I can get better A-B-A data.
Along with that, I replaced the tube-and-fin heater element with a plate-and-fin element. Instead of a 45 F differential between outside and intake air temperatures, there is now about a 60 F differential. It reflects favorably on the test drive, too. On an all-city driving test run of about 8 miles (between my home and my parent's house), I am estimating a 15% improvement in fuel economy. (That's all in-town miles, there. My commutes will doubtlessly show a smaller increase than 15%.)
The indifferent news: I did not test the heater shutoff valve, because I wanted to do a leak test of the reworked heater hose plumbing.
Now, the bad news: All that A-B-A test data above? Say buh-bye to it... I'm starting over from scratch.