1) it might be a CPU issue, or the traces/solder joints may be broken. If not, then the power supply might not be outputting on some of the PSU-Motherboard connector's pins. The condition of the boards is also imminent to the functionality of them; Corrosion, trace damage, general time wear on the components, these can (and will) affect the functionality of the PlayStation.
2) Being a professional in the field, I'd highly suggest starting with the Power Supply Board, and go from there. Just something experience has taught me. As for schematics, you can get your hands on the service manual, which has everything you will need and more. However, actually obtaining a pdf of it is difficult, since Sony forbade the reproduction and public distribution of it to non-affiliates and unlicensed personnel. But if you search around, you can probably find it in some shady or foreign website without use of the Dark Web. I found a decent Pdf of it, but I can't link you to it since I'm writing this on my PS4 and don't have the ability to copy or paste. Search 'SCPH-(your model) Service Manual' on Google — without parenthesis, of course — and see where that gets you.
Best of luck.