well this was a productive waste of time: i managed to replace the 2kOhm charger identification resistor in the plug with a 1kOhm one (make it identify as a 135w instead of 170w) because i thought that would help, but it didn't

what i did find out however, is that while this charger happily keeps the laptop on at high performance (discrete GPU on, high brightness.. haven't run loads yet though, if i plug both AC and battery in with the laptop off, the LED in the AC charger starts flashing...

not any logic flashing but a clearly-something-wrong-with-the-power pulsating, but since the brick happily powers the pc alone, i'm betting there's something bad with the battery charging circuitry in the laptop...

i bought a secondhand legitimate 135W lenovo charger and it won't even power the laptop on by itself. the knockoff one, however, does

(neither will charge the battery)

i dunno how to solve this. nobody else on the internet seems to have this issue.

That chip looks unhappy. And it's epoxied. Great!

yep, definitely the culprit. i just saw it spark and now it smells very burnt