Just a double-check for my own clarification:

a) you have no display, even at boot.
b) you get the flashing standby LED as normal
c) pressing and holding the top botton gets rid of the standby LED
d) you can get no sound out of the player, even when tinkering with the controls
e) you can sync correctly and still see your data

Hmm... What if the circuits controlling the four buttons, or maybe just one of the bottom three (no knob on the Mark1) were somehow broken and sending gibberish? If shorted, you might have lost a fuse or fuses, but the button may think it is constantly pressed, flooding the player with a stream of commands.

Does anything look strange on the display board itself? Can you unplug the display board and (using hijack or serial commands to remote control the player) hear music out in that state???