First off, thanks to the Wednesday Group for encouragement and bug reports.
Bug : It's not recognizing all my key presses
After testing, it seems this is a problem with the implementation of key reporting in various keyboards. I read the USB spec once and it seemed to imply that you're supposed to be able to correctly report up to 8 simultaneous key or mouse events. I have a cheap Amazon Basics keyboard that doesn't quite do this. And a friend reported an older MacBook wasn't delivering all the key events required.
So... you may need to try it with a different keyboard. But please let me know if you encounter problems with the mock-up recognizing keys. My contact info is on the About the Author page.
Enhancement : What about the other hand?
I'm an oldster; I mouse with my left hand (even though I'm right-handed.) I was kind of surprised to find how many other people over 40 do the same thing. Seems like once you flirt with tendinitis or carpal tunnel, it scares you into mousing off-hand.
However... I get it that some people mouse with the other hand, so adding a left-hand / right-hand radio button to the interface might make the mock-up more useful. Maybe using the keys SPACE, F, D, S and A in "left hand mode" and SPACE, J, K, L and ; in "right hand mode".