A guest user may be able to make changes, but those changes are lost when the guest logs out - reset to user defaults.
Guest can make any changes they like, but all changes are dumped when the guest logs out. The next user to log in to the guest user sees only the defaults.
The guest user can't unlock system-wide settings, unless the guest knows an admin user and password for that Mac.
Guest should remain a guest. IMHO, if they know the admin credentials, then they are no longer a guest, eh?