Z key not working, replaced everything and still fails. Help.

So, full description of the problem first: When any disk in any mode is booted up on my MBP, the Z key doesn’t function. However, when it’s first turned on, when you first hit a key, it always types Z first, unless you hit the Z key first, then it just keeps typing Z like your holding it down until you hit another key. This effects my startup commands as well, so far all I can do is an SMC reset, go into single-user, and disk recovery, but that’s it.

So! Where it gets really weird is what I’ve tried so far. To date, I’ve:

• Reset SMC and NVRAM (several times, NVRAM from single-user)
• Swapped out multiple keyboards
• Replaced the Trackpad cable (just in case)
• Put in a new disk and installed a completely fresh copy of Mojave, then tried Mavericks for kicks, just in case.
• Stuck another disk in that was installed with OSX from a completely different early 2015, one with a working Z key.
• Checked and messed with pretty much every computer settings file that has to do with a keyboard

At this point I’m at a loss. There was no liquid damage on the device, and no other damage. I really don’t want to replace the mobo, and I’m down to solder or replace anything on that board to get this working, but if it was a trace that failed, I would assume at least that more than 1 key would be effected, since they are grouped, save for left bottom 3 and caps lock. I am officially out of ideas. Help.