I don't think a failed flash should cause the board not to power on at all. At least, I haven't heard of a failure like that yet.

indeed usually it will power on with the fan at full blast or something and not much else.

Capacitors: as long as it is not KZG or KZJ should be fine then. Nichicons have been reported too on these boards. Not the caps. Back to the drawing board.

I'm starting to think a cold solder joint on the green wire pin or something.