Removing and reinspecting the ECM board showed now other obvious signs of failure. The apparent cracks on the this transistor proved to be an incomplete coat of acrylic.
After removing the ECM and control fuses, I went through the ECM connectors in the harness and carefully bent each pin so it would have more force to grip the main board, and I sanded the contacts with emory cloth. Sadly, this brought no joy, so I caved I purchased a refurbished ECM. My research showed that the ECM used in this car would work in something around 300 GM models between 1985 and 1995 (ish), with all behavior dictated by the ROM that was placed in the board.
Opening the ROM bay of the refub unit showed a mainboard that looked worse than the one it would replace. It had multiple layers of lacquer, visible corrosion, and an obviously older design that featured many more through-hole components.
Compared to the semi-function ECM, which was all nice and clean and loaded with surface mount devices.
A peak inside the working unit:
Swapping the ROM was easy. It turns out the holder is keyed and designed to support the pins of the chip. Unless done without any regard to care, and perhaps using a hammer, exchanging it is very easy.
Unamazingly enough, the car works great with the new ECM.
I considered, for a bit, trying to reflow the flakey board once I had the replacement in hand, with the mindset that I could send it back and reclaim my ~100$. However, after spending so much time trying solutions that didn't work, it seemed silly to ride that particular merry-go-round again. There had been some musing about using a toaster oven or heat gun to reflow the board, but after the car was running again, stinking up the house with particular activity seemed a bit silly.
Instead, I celebrated by changing the oil.
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