A1502 trackpad unresponsive and keyboard occasionally erratic

My computer is a mid-2014 retina MacBook Pro (A1502). It's well out of warranty, so taking it apart and replacing components is alright with me.

After five days of sleep, the trackpad stopped responding to touch input, and couldn’t be revived by any number of NVRAM/PRAM/SMC resets. I've checked with FingerMgmt and found that no data seems to be making its way from the trackpad to the OS, indicating a hardware fault to be most likely. Most recently, the keyboard has also started to become unresponsive after waking from sleep until the computer is rebooted. No water damage occurred.

I have access to some spare parts due to a previous issue with the trackpad cable:

- 2x iFixit trackpad (excluding cables),

- 1x iFixit trackpad cable, and

- 1x original Apple trackpad cable (known defective)

For clarification, in my attempts below, I refer to the Apple trackpad and cable as “original”, the two iFixit trackpads as trackpads “A” and “B”, and the iFixit trackpad cables as “1” and “2”.

The setup currently operating inside the MBP comprises iFixit trackpad cable “A” and the original Apple trackpad.

I’ve tried the following things, to no avail:

1. Reboot

2. NVRAM/PRAM reset

3. SMC reset (NVRAM/PRAM/SMC were reset multiple times now and after almost every later step)

4. Checked that trackpad is still recognized under System Information – still there, from previous experience if the trackpad isn’t detected it doesn’t show up in here

5. Deleted trackpad preferences

6. Tried updating software to latest version – macOS Catalina 10.15.6

7. Tried `diskutil verifyVolume` and `diskutil repairVolume`

8. Tried booting into Recovery to isolate if issue is software or hardware related – seems to be hardware related since the issue persists in Recovery

9. Ran Diagnostic Test to see if any issues with trackpad were reported – no issues were found

10. Tried booting into external boot drive to isolate if issue is software or hardware related – issue persists

11. Tried original trackpad with original cable – this is a known defective combo, which gave me trackpad issues last year with wild inputs; no inputs were detected this time around

12. Tried original trackpad with iFixit cable A

13. Tried original trackpad with iFixit cable B

14. Tried iFixit trackpad A with original cable

15. Tried iFixit trackpad A with iFixit cable A

16. Tried iFixit trackpad A with iFixit cable B

17. Tried iFixit trackpad B with original cable

18. Tried iFixit trackpad B with iFixit cable A

19. Tried iFixit trackpad B with iFixit cable B

20. Debugged in Single User Mode and saw errors arising from AppleUSBMultitouch saying `[HID] [USBMT] [Error] ReportID is invalid in AppleUSBMultitouchDriver::_deviceGetReportWithLookup` – I doubt this is the issue, since it should be driven by `AppleMultitouchDriver`; but as the trackpad is classified as a USB device under System Preferences, I wasn’t too sure

21. Booted into Recovery and saw that the error message mentioned above did not appear, arousing suspicion it is software related – again, I’m not sure if this is the issue; even though the error message didn’t appear the issue persisted

22. Tried replacing `AppleUSBMultitouch`, `AppleMultitouchDriver`, `AppleTopCase` kernel extensions with fresh kernel extensions from macOS installer – didn't work because codesign is different

23. Tried macOS 10.15.6 reinstall – also didn’t work, but the error mentioned in (20) was no longer present

24. Tried fresh macOS install on a brand new partition – no luck here either

25. Procured replacement iFixit trackpad cable, and tested with original trackpad – this was done on the suspicion that all my cables may be defective

26. Tested replacement cable with trackpad A – didn’t work

27. Tested replacement cable with trackpad B – didn’t work

28. Reseated trackpad and keyboard cables fully – didn’t work

29. Removed logic board to inspect CY8C24794-24LTXI USB controller connections for corrosion – I live in an area near the ocean where corrosion of metallic parts is inevitable after a few years, so I figured that might’ve been an unfortunate cause. However upon close inspection with a microscope looking for signs of corrosion, dejointing, or shorts no glaring issues were found with the chip or its joints. The ZIF connectors were also in good shape.

I’ve also taken it to an Apple Genius, but they ran their tests, found nothing wrong, and told me I’ve already done pretty much what they would’ve done, if not more. This leaves me quite stuck with truly no idea what’s going on with my trackpad and keyboard subsystem. For now, I’ve started using an external Magic Trackpad 2, but it doesn't beat the convenience of the internal trackpad.

Are there any possible known causes of this issue, or perhaps solutions to reviving the trackpad? Alternatively, is there anyone from Apple, perhaps an engineer, who I can speak with and would be able to point me in the right direction to rectify this issue?

Thank you for your time.

Harry