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Right, so , after a brief hiatus where life got in the way of doing something of interest, I'm back.
At this point I'm 90% certain that my screen is not broken and 80% certain that the TPS65285 PMIC chip is faulty based on the boot error messages and the voltage measurements.
However, the chip has a substantial number of external support components - diodes, resistors, capacitors and inductor coils any one or more of which might have failed and be responsible for the chip failing to fully power up.
I can limit myself to just investigating the external components attached to the DCDC2 and DCDC1 subsystems: the former needs to be working prior to powering up the latter. Together these two provide the power rails for the EPD function of the screen.
This exercise is based on the assumption that the typical application circuit in the TPS65185 datasheet is what is implemented on the Kobo motherboard. As it turned out this is almost true, the Kobo also has some extra filter capacitors beyond those indicated on the diagram.
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