Looking through the schematic of the disk controller, it wouldn't surprise me if the bad inverter also ended up causing the parity errors. That one pin ends up affecting error detection, clocking, and addressing of the disk controller, which could very well end up manifesting as the parity errors you're seeing, especially given that your entries suggest that the errors went away once you dead bugged the inverter. Regardless, this is definitely an interesting series and I wish you lots of luck in getting things running.