MacBook Air Early 2015 Trackpad pointer not working

[cb27]

OK, here's a weird one for you all.

So someone brought an 11" Early 2015 MacBook Air in. Said it had previously had a new keyboard and trackpad after liquid ingress and everything had been working fine. Then the guy's daughter spilt some water on it (again!) and since then the rest works but not the trackpad.

So I tested it:

1. Apple Diagnostics finds nothing wrong (No issues found ADP000).
2. A replacement trackpad yields the same symptoms
3. The keyboard works fine with both trackpads which is kinda weird as it's cabled up through that stupid upside down cable that goes via the trackpad. You'll know the one I mean if you've ever disassembled one of these.
4. The click works, but the mouse pointer doesn't move. And yes I checked all the Accessibility and Mouse/Trackpad options to ensure it's not being disabled
5. The pointer doesn't move using the trackpad but does with an external mouse or with mouse keys / head pointer (Accessibility).
6. The behaviour is the same on the Apple Diagnostics screen as well as an external boot drive - different macOS version
7. There are no trackpad related plist files to delete which was another suggested fix

I would have thought the circuitry on the bottom of the trackpad would shield the logicboard from damage and the fact that the keyboard (which is wired up to the logicboard via the touchpad) works flawlessly would indicate that the connection from the TP to the logicboard is good including the kbd/tp interface chip on the logicboard?

The only two possibilities that come to mind are:

i) The replacement tp has the same fault. Highly unlikely
ii) The logic board has a fault that affects only the trackpad's ability to control the pointer and everything else works flawlessly. This still seems like a remote possibility to me, but I don't know enough about how the tp & kbd signals are multiplexed through that narrow cable to the logicboard to say whether this is a plausible explanation.

Have I missed anything else I could try?