Well, the short answer is that I got this thing working... and that it needs a new button panel... still.
The long answer is that I wanted to confirm that the faulty part was indeed the button panel before I bought a new one for $100 +shipping.
So I did some reverse engineering and used some old arcade parts.
This is how we do it "In the Ghetto":CLICK ON ANY PICTURE BELOW FOR A LARGER IMAGE
I ended up only using 4 buttons: Speed up, Speed down, Start and Stop.
I had planned to use the 1 and 2 player buttons for 3mph and 6mph, but frankly, I was tired of it after deciphering the first 4 buttons. CLICK ON ANY PICTURE FOR A LARGER IMAGE
The hard part is deciphering the button panel. It turns out each button press results in 4 different wires together. I assume this is to reduce the number of digital inputs to the controller... but it made it super hard to figure out how it worked. I ended up tracing the circuits on the button panel and following them back to the wire connection... tedious (and reminiscent of TRON
but the boss is happy now, because her treadmill is "working". And I think I can now safely order a replacement button panel.