Most mindboggling problem to date: Phone won't power on

I’m a seasoned troubleshooter fixing everything from computers to smartphones to consoles and so on, and this one has me stumped.

The phone is completely dead, doesn’t turn on, doesn’t charge.

Things I’ve already ruled out (and why):

1. The battery is fine (tried multiple batteries and swapping and charging both
2. The charging port is unknown (because even without charging the phone should work with a fully charged battery
3. There is no water damage, as seen by the indicator which is white.
4. The power button isn’t broken, because the copycat phone (identical but not the broken one) turns on automatically when charging.

So this leads me to believe it has to be:

1. the motherboard, as that’s the last thing that could be preventing it from powering on.

1. The charging port, still not completely ruled out.

Please help, this is driving me insane.