Eric: "It is evident from the analyses here that calorimetry over a phase change is problematic and that a change of phase should be avoided .... Question: this point seems clear enough that I suspect it might be undergraduate-level knowledge for anyone who learns about calorimetry in college. Is this the case?"
It is correct and what everyone has been saying since Rossi's early flawed phase change calorimetry.
Setting aside the question of whether a test of a 1MW device ever made any sense at all, let's assume that a phase transition is required for a practical device. At the cost of reduced accuracy, presumably one can still avoid phase change calorimetry by using a heat exchanger with a second fluid that does not undergo a phase change. So boiling water does not by itself defeat a test. But if your reply above is correct, and it is indeed college-level knowledge that phase change calorimetry is problematic, a question to be pondered by anyone who cares is why phase change calorimetry (without a heat exchanger) has been used in so many of Rossi's tests. Using it for one or two tests, ok — live and learn. Using it repeatedly: this is indicative of something pathological.