I've actually gotten pretty far with the above by copying the Estuary skin to a new name, then ssh-ing in to the new skin and blowing away the stuff I don't want (using the above keymap to help get around on the skin side) - a lot of things I didn't want got turned off in Settings, then Settings itself isn't accessible (other than through the temporary keymap) due to the removals of settings access points in the skin.
This is definitely more of a one-off project than something easily bundled to ship for others to use, but for people who can get around an ssh connection and edit xml files, it's not too hard to do.
So, with a combination of settings, skin removals, node removals, and keymap mods I think it's fully secured. Time will tell.
Help I got from the Kodi forums is in this thread: https://forum.kodi.tv/showthread.php?tid=376699