After sleeping on it, I made one more attempt today at trouble shooting the issue - this time, 100% success. Phew.
It turns out that I had [rather stupidly] reconnected the trackpad ribbon incorrectly, by feeding *under* the white pressure clip instead of above it. So simple (as it usually always is!), yet so fatal.
The new keyboard is working beautifully now (although the down key is a little sticky, tho I'm not complaining... I'm just glad it works!). It almost feels like I have a brand new laptop!
We now return you to your scheduled programming. This case is officially closed.