It was corrosion on the sub-harness spade terminals; specifically, the ground spade. Even though the corrosion was uniform on all terminals, only on the ground did it prevent any continuity to the PTU; connection to the coils were not adversely affected, with ~1.5 Ohm resistance. Scrubbing with a wire brush followed with electrical contact cleaner and I got ~2 ohms to the PTU for ground. Reassembled everything and the car fired up the first crank! Now I have a single misfire to resolve, but dang, I'm glad I was able to troubleshoot this on my own, allowing to learn more intimately the ignition and the electrical system in general. When I replace the entire EFI wiring harness as I replace the turbos, that should do away with any further electrical gremlins!
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