Also - if that seems to show signs of life, you might need to remove R38 & R37. They connect the SPI flash pins to the SAM3U, which even if held in reset means there are 4-5cm of hanging trace that will limit the SPI speed. I think you can configure the SPI speed as part of the PROM file generation, so you could crank the speed way down too. But probably the chip should be detected at least…
