easiest way to to flash latest available rom for your region/carrier using odin, then root using towelroot. i'm using the latest uk btu which is still FNB8. I've tried loads but always seem to come back to this setup, it's the most stable for me.
If I'm understanding you correctly, you want to be able to root/unroot at will?
Easiest way I found is if you have custom recovery Philz. With this you can go into Philz setting, restore root and then install SuperSU from store. Then to unroot just choose "Full unroot" in SuperSU app and you're done.
You can also use twolroot to root the phone then install SuperSU to full unroot.....although I've found that this can take a couple of attempts of rooting/unrooting before the UNROOT will hold for some reason. At the moment I'm on totally stock ROM, stock recovery, but I rooted with towelroot to make some adjustments using xposed/wanam and to edit the snd_soc_msm files for louder music and remove bloatware using rootexplorer, and have now finally got unroot by rebooting/clearing cache at each stage of the process (i.e. when i root, when i install supersu, when i update the binary in supersu, then i reboot one more time once root is confirmed working, then full unroot and uninstall towlroot app and reboot/clear cache once more).
So now I have a sort of custom stock rom without root, sky go player working. Perfect setup until I get brave and a new update comes out.
there are also pre-rooted stock roms in this thread,
not sure if the procedure for unrooting would be the same though? or even if you can using these roms?