How can I detect that a ZVD-style 'backpower protection circuit' is operating correctly?
The circuit is correct, but your problem is only that the design parameters are not-defined for Vt, gm, β,Rs,Rc or RdsOn and thus the wrong load was used to simulate a power supply.

Here  I chose gm = 5 A/V above threshold with Vt = -2 V

With high impedance ratios for gain of Rc/Rs the FET starts conducting with a forward voltage of about 2% of the Gate threshold voltage.





High values of collector R achieve a high differential voltage gain on the FET using the battery voltage to bias the FET and the ratio values on the PNP collectors of 1:5 yet very high ratios relative to battery impedance to reduce the reverse current to 10nA.