I am sorry if this is a stupid question but when you say data/local.prop-do you mean root/data/ or just data/ in internal memory?
and if I create local.prop and add the line qemu.hw.mainkeys=1 in local.prop would that be able to hide the navigation bar even if the I don't touch build.prop?
It does go in /data/ and you can simply make a new file, name it local.prop. Any lines you put in local.prop that are not in build.prop will be used just like in build prop. Any lines you put in local.prop that are in build.prop will take priority (so you dont have to change any lines in build.prop) unless the line in build.prop is ro. (read only). To circumvent this you will do as you suggest by adding qemu. if that line in build.prop starts with ro.