It does sound like this has stopped working normally somehow, as opposed to hardware that isn’t supported by Linux. This thread details quite a bit of troubleshooting that has gone into getting various bits of the laptop working, and it seems like no one has the same issue with the power button: Macbook Pro Late 2013 Retina / Laptop Issues / Arch Linux Forums
The power button is mentioned in this device’s ArchWiki entry (User:Elinux/MacBook Pro 12,1 - ArchWiki), however it is in the context of changing what the power button does when pressed–not getting it working in the first place.
I have to wonder if the connection was disturbed with all the opening and closing of the case/plugging and unplugging of the battery. You may be tired of opening up the case by now, but it might be worth examining the connection of the power button to the motherboard. Double-check the connection is well-seated. Try disconnecting and reconnecting, if the connection is not a solder.