prodded the keyboard with:

I did a bit of a detailed examination of the board with a magnifying
glass for any possible damage.

I found something on the back of the board that I hadn't noticed
before (probably because of the blown part on the front). It seems
like there might have been a short between a resistor (R20) and a
diode (D4). I have taken some pictures and added it to the album
(https://imgur.com/a/uBCXjwQ).

Not sure if this helps or makes any difference, but let me know if
there is anything else that I can check/try.

Looking at your pictures, the transformer has been very hot at some
time ! Suggesting a heavy load for some period of time.

--
Best Regards:
Baron.