hello there,
you've already discovered a lot and but... start from the beginning. You need to determine if the motherboard boots. Remove the matrix and connect an external monitor. Correct me if I remember wrong - this model already supports video via usb-c port (displayport 1.2). If so, follow the path (white rabbit :-). Be sure to check the values in the screen port. It would be useful to determine if the screen works. If you have another computer, or have the ability to check with another computer, it would save a lot of time.
Check presence of 3V3/5V_S0SW_LCD and DP_INT_HPD while lcd connected (pp5v_s0sw_lcd & pp3v3_s0sw_lcd)
that's all for beginning.