Update: Actually, with slight tweaking, the redirect via URL does work perfectly well, and I am able to mask the subdomain URL remotely, but in doing so I lose page IDs and all pages display the root URL, and this obviously is unsatisfactory.
I did try changing the site address of the subdomain to the root in General Settings, but this breaks the site. At this point I would like to know how to manage the appearance of the URLs with this configuration, and I would still like to know what if anything should be done with the root site, so that it is no longer accessible.
