It looks like you already know about commanding DiagROM to run in Fast RAM by holding the left mouse button at power on (releasing it before DiagROM checks for stuck buttons) - very handy if Chip RAM issues are preventing it from running otherwise. Note that this does require a trapdoor (or other) Fast RAM expansion to be installed. There will be no graphical display, so you'll need to monitor the serial port (as you've been doing) for output. If you also hold the right mouse button it will use low Fast RAM ($00C00000).
I'd probably keep it at 512 kB (with the default jumpering) for testing. Does rearranging the memory chips change the pattern of the errors? Do the resistor packs test OK?
