MAS super pro stick mod

I have an MAS super pro stick for gamecube, that I want to put a PiiWee board in. I would like to keep the GC support, however I don’t quite follow the wiring or know how the MAS chip works.

What I think I know:

The buttons on the left are using the com terminal as the signal wire and going to the corresponding pin on the chip. They are using the NO terminal as their ground.

The joystick appears to use the com terminal as a ground and the NO terminal as the signal. The two purple buttons on top use the com terminal for the ground wire from the joystick’s up switch, and the NO for signal.

What I don’t know:

How does the switch at the top left work? It is used to switch the joystick from analog to dpad mode.

Why the switch between which terminal is ground? Can I just solder a wire from the top left purple button’s com terminal to one of the PiiWee’s GND points for its ground? Then a wire from each buttons signal terminal to the matching pin on the PiiWee?

I’ve never done anything like this before, so thanks for the help.