If I jumper the PSU-ON all PSU voltages are good (obviously no load). I have 3V power to the on/off switch pins on the motherboard. I've cleared NVRAM, reseated RAM, tested the RAM in another machine, reseated the CPU but it won't power on. ...
Your MiniToolBox report is showing lots of Nvidia display driver errors, so you might want to try a clean reinstall of the drivers, as follows (this won't fix all your system errors, but could be a good start):