I note from the links provided by both you and Art on LH failure and repair the following symptom is listed
o Automatic Transmission: no RPM regulation (idle speed) when switching from "D" to "N" or "P".
So it seems you’re on the right track as that very much agrees with your clutch load/noload observations, although there’s no explanation of why that happens.

Like you say, the ECU can only see (deduce) the clutch pedal, brake pedal and shifter indirectly through the CPS rpm and AMM air flow and maybe the TPS to suggest your foot is also off the gas pedal or the speed sensor to suggest you’re at a standstill.

Maybe on the board under certain operating conditions some failing circuit is leaking out into the ECT circuit nearby, which would directly affect base idle. At this stage for you, I’d want to try opening up the ECU, inspecting for hot spots, cold major solder joints, cleaning the solder side (like with a toothbrush and circuit board cleaner or 99% isopropanol), using compressed air on the component side, maybe even flushing the component side (like with spray circuit board cleaner, contact cleaner or non-residue spray AMM cleaner).

As for the evidence you saw of a PO having replaced the ECU in your 245, although I would expect the pink label -561s were out of the supply chain by 1992, it would make you suspect it was originally a pink -561 with someone having previously bypassed the failed fuel pump control circuitry (which the pink label ones were infamous for) as a temporary fix until they could find a white label one.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now