The board was fully cleaned with pure Isopropanol on both sides. I checked the complete PCB under a magnifying glass, didn't find any issues.

I also tried the procedure of quickly sliding it in and out with the power switch on, multiple times to no avail.

Eventhough I measure power from the Pram battery on the ADB ports, it does not start.

Maybe the ADB chip or PMU is dead.

I tried replacing the Roms as well from a spare known good set to no avail as well.

I got the board from a fellow collector in non-working condition even before recapping, so I have no idea what happened with the board before I got it.

It look beyond repair as far as I'm concerned. As far as I know there is no other way to power it on as through keyboard.

It's regrettably not like a Mac IISi or IIFx that has both keyboard start and a power button at the back.