Are you sure the ribbon is inserted the correct way with contacts on the correct side? You can insert a ribbon wrong and nothing will happen. Try inserting ribbon the other way and it might fix your issue. One side has contacts and other side is just plastic backing!
If this isnt the issue then continue reading further...
Given that a short to the 2 pins boots it, its obviously the microbutton switch assy, given a motherboard replacement would have swapped the ribbon lock connection contacts.
If your able to take a multimeter and set it to continuity with a tone. Touch leads together and you should get a tone. Next hold carefully without damaging the test leads to the contacts of this ribbon disconnected from the main board and have someone press the button switch and if you don't get a tone then the issue is these replacement part are defective as a tone should be heard when momentary microbutton is pressed in.
If unable to find a good source for these replacements, if your skilled at soldering small connections, you could run wire wrap wire to the leads on the motherboard to the 2 contacts of the microbutton switch and as long as the momentary microbutton is good it will fix the issue. You may need to run 2 or 3 more wires if the push button has a LED that lights that is single-color or bi-color, and polarity would be needed to be known if you need to run those connections as well as for wrong connections with LEDs = LED wont light or LED will light with the wrong color if wired wrong pair on a bi-color Amber/Green etc. * Additionally there is a 2 lead type of bicolor LED as well that can show wrong color if wired backwards ... Bicolor LEDs are usually a white or clear color body to them and if unsure the troubleshooting guide for your laptop would be a good place to look as for it will state amber for a fault condition or a mode condition and possibly green or white illumination if powered on ready to run.
Here is the wire wrap wire that I use for running new connections for situations like what you have with very low current connection repairs.
https://www.digikey.com/product-detail/en/adafruit-industries-llc/1446/1528-2005-ND/6827138&?gclid=EAIaIQobChMIgNrb6bjJ4wIVRsDICh3o9gCPEAQYASABEgL-UvD_BwE