Trackpad and keyboard mostly unresponsive even with replacement ribbon

Hello!

This component seems like a super common point of failure on the Early 2015 MacBook Pro, however the typically recommended repair procedure doesn’t seem to be working for me.

My keyboard and trackpad have been unresponsive for years and I finally decided to go ahead and replace the ribbon connecting the trackpad and keyboard to the motherboard. Unfortunately, this has not resolved my issue. I’ve tried all the typical stuff (resetting PVRAM, SMC, etc.) and nothing has worked. Keyboard does not work when I boot into Ubuntu, however the keyboard DOES work to initiate an SMC reset.

The hardware must be working because system recognizes CMD+OPT+SHIFT+PWR for SMC reset before boot. After boot, keyboard does not work for PVRAM reset, booting recovery mode or diagnostic mode, etc. Keyboard does not appear in list of USB devices.

Also strange: the trackpad does give haptic feedback on click once the system is booted up, it just doesn’t recognize any input.

I’m guessing I will need to install a new trackpad, but is there any reliable way to know for sure if this is the issue at hand? I don’t see any corrosion on the board - I do see some brown residue on the inside of the laptop’s shell. I’m quite upset that I may need to replace the trackpad, as I literally just replaced the battery at the same time that I replaced this cable and it would be a waste of money and lithium to need to install a brand new battery again just for this.

Thank you in advance to anyone who offers help! I’m excited to get this darned thing fixed. :)

Pics below.