1. Never. It doesn't help that they're hidden behind fn-← and fn-→ on my laptop. Years of Emacs usage acclimated me to using ctrl-a/ctrl-e and on the handful of programs that don't support it (like Word), my first step is to make them new shortcuts for that app.

2. Yes, but what should go there? One of my first steps on a new Mac is to go to keyboard/modifier keys and disable caps lock.

3. Yes, although I'm noticing that I favor my left thumb for spacebar usage. I think it's wide because of the variety of ways people use it.