Lots of work there, so I tip my hat to you, sir. But you might want to check out programmable keyboard encoders- they require a lot less soldering and you can assign each key whatever function you want. I'm typing this on a 1200xl right now.
I've successfully cleaned all 7 of my 1200xl keyboards and found it easier not remove the mylar completely from the pcb. Instead, I used a paper binding clamp on the contact area, which allows you to separate the mylar far enough from the pcb to clean all the key contacts while leaving the connector contacts intact. On one keyboard where I did accidentally separate the mylar completely, I was able to get it working again without using conductive paint by sticking a bit of electrical tape above the contact area. The thickness of the tape added just enough extra clamping force to make the contact.
I like the idea of using the atarimax usb cart. I wish I had the programming skill to write a handler to do the conversion. You can also use a universal programmable ps2 or usb encoder (e.g., Hagstrom Electronics) hardwired to the Atari keyboard. That's what I'm using right now with my 1200xl.