If Hardware buttons or capacitive buttons dont work!!!!
This problem is not common.
But it is major problem, because it is an hardware.
It can be overcome by having similar tool (app) which will function like hardware buttons.
It can be accesses by notification bar.
Icon will look like logo of creo.
It will float on window and can be move within whole scrren according to our wish. Not only buttons it can work for cleaning background apps though and accesses to recent or our fav apps.
OR just give the option to enable the navigation bar.