1. Without knowing exactly what counts as an "end-user", 15%. This is low for two reasons, 1) because I can SSH into a plethora of non-graphical systems, which I assume are the majority; and 2) just to be difficult and pedantic with the new Wayland systems for example.
2. Should be like 95% ;)
3. Without ever having used KeePass, I'm quite confident of the answer to this one... 100%. Is it officially supported and easy though?
4. Also 100%! But not happily.
Sorry, without some additional context I can't really give you satisfying answers I fear.