Amazon seems to have learned from 3rd/4th gen gaffs as 5th gen devices offer a more robust recovery menu that supports reloading FireOS. Many HDXs with simple os/rom malfunctions (eg: bootlooops) were fundamentally unrecoverable due to the missing capability. Kinda short sighted on Amazon's part.For anyone else with this challenge, I restored my backup and used super-sume, then I removed SS recovery deleted any of the remaining Kingroot directories and finally used the uninstall option within supersu. I confirmed I didn't have root any more so I guess its similar to a 'clean' tablet. Its a shame Amazon didn't cater for a re-install of the OS with a fresh version. It wont do an 'upgrade' if it doesn't detect a version change.