Legend of keys and disabling one mouse.

There are a couple ways to do this. You can program a key as "Mouse Disable" (be sure it's not the "Both" version) on the hand you want to disable, and then press it. The mouse should be disabled after that, and you can re-use the key for something else. Another way would be to program the "Mouse Motion" as "No Action" for the hand that you don't want mouse motion on.

Neither solution worked.

Have you saved your layouts to KeyMouse Devices? Both methods should work fine.
Note: when saving to KeyMouse Devices, you will need to program both hands (the software is only capable of programming the 1 hand plugged into the computer so you will need to plug the other hand into the computer and program it as well, until the next firmware update).

Here's the config I saved to the left mouse.

I just tried that. Still didn't work.

Notice the "Not Set" next to your layout "Qwerty - Custom" in the screenshot. You will need to click that and set that layer to your white layer. It's not saving that layout to the device currently.

Hello!

Thanks for that. That worked!

Excellent.