Ah you already got fan and mobo out
There can be some oil spils etc on the heatsink its ok, only if you see liquid damage or corrosion on motherboard that is bad.
Need more high resolution photos of motherboard. Did you see any damage or liquid in motherboard?
Try boot with only fan connected and power button. Remeber when using power cable keep motherboard in non conductive material like wood or cardboard.
Also if you remove cmos battery and plugin power cable it should boot automatically without even pressing power button. This is good for testing if power button cable / board is fine.
Try also test without fan connected at all. does cpu get hot when trying to power on? Take power cable fast off if it starts getting hot so wont damage cpu
Do you have a spi programmer or multimeter?