I don't know if my answer applies to your specific version of plasma, assuming of course that you are still using the same version a year and a half later.
In newer versions of Plasma the 'Keyboard Navigation' tab appears to be under 'Accessiblity', not 'Input Devices'. It is now called 'Mouse Navigation'
You can turn on Mouse Navigation from the shell using the following command:
kwriteconfig5 --file ~/.config/kaccessrc --group Mouse --key MouseKeys --type bool true
or by editing the
~/.config/kaccessrc file manually, as you did, to set
This will enable keyboard control of the mouse when you next log in. If you want it enabled without having to log out, run the following command, which takes 5–10 seconds on my system:
kquitapp5 plasmashell && kstart5 plasmashell &
& is important as it starts the process in the background and you won't kill the plasmashell if you close the terminal emulator you used to run the command.
kquitapp5 ... command was given to me in answer to a question I asked about a similar topic.