The PSU works, that has been confirmed.

I've doused the affected area of the board with 7% vinegar, and after some bubbling and sizzling, rinced it with vinegar, scrubbed the area and used a toothpick to pick the last bits off. Then some alcohol and I left it out to dry in the early summer sun.

It looks better than before:

So is it working now? 😀

I actually suspect more on the tantalum caps than the corrosion, even though the corrosion is real bad. Usually when there is a short, the PSU should shut down instead of tripping the circuit breaker. In your case that didn't happen, so perhaps the quality of the PSU is dubious too. It only happened to me once when fixing a cheap old PSU. The manufacturer substituted the Y safety caps with normal ceramic caps, and the ceramic cap failed it caused a short from live to ground, tripping the house breaker.

P/S: some of the traces beneath RP22 looks affected. Traces under the ISA slot might be affected too.

Good point on the PSU, I suppose it's far from reliable. With another PSU, the power does not trip.

The system does not boot, though. Depending on which ISA slot I use, the output of the test card is either

I've only been able to quickly test the graphics card on an ASUS ISA-486, and that is ok. I'll test the ram from the 019R1 later today.

If you have a multimeter you can do a quick check on the power rails to ground to see if there are any shorts.