I highly suggest going over the pins and contact plates for the display and battery connector. I had the same issue with a repair and it turns out one of the pins for the battery contact board ended up being scorched.
"Curiously I have replaced the motherboard with a known good variant and it is still refusing to turn on. Two different screens and two different motherboards. The only commonality is the power switch and the battery.
Battery appears to charge adequately. So I assume it is functional.
Did this ever get resolved? I've got the same issue after I removed my screen. The shim is on the power connector. I get no power to the device whatsoever.
I think the answer lies in a mosfet blowing. But without the schematics it is very difficult to tell. I've tested all the resistors and fuses on the boards that I have and they all seem to be conductive, although of course without schematics I cannot tell whether there are any that are partially failed and therefore at the wrong value.
If anyone has copies of the schematics online I can test further."
