Your first concern seems like a non-issue. Assuming your fingers are always making the same movements, it's effectively like the F and J are magically wherever

Your first concern seems like a non-issue. Assuming your fingers are always making the same movements, it's effectively like the F and J are magically wherever your index fingers happen to be. "Can't reach the whole keyboard" is a bit of a problem, but the obvious way it would work is that the signal which the computer interprets as "position my hand over the numpad" becomes a input mode shift. There's probably more there. Without knowing anything about how the sensor tech works, I suspect that there's a difference between the signals when my hands are on home row and when my right hand is on the numpad. The mouse seems like it might be even easier in the long run. I'm assuming the training is harder, but waving hands in the air sounds way better than resting hands on a padded surface. RSI in general sounds like the X factor. Input via odd muscle twitching sounds like it could raise all sorts of weird muscle issues long-term.