I followed precisely the instructions in https://premium.wpmudev.org/blog/how-to-safely-move-wordpress/for moving a website from a subdirectory to the root. I used the set of instructions for keeping the site on the same server. For example: moving dev.x.com to x.com, so visitors would access the site by referencing http://x.com.
It didn't work. After the move, I could see only the home page. All other links got 404s. The index.php that I modified is attached, as is the htaccess file that was in my dev directory, and which I moved to public_html. I made no change to it.
I raised this question on today's chat. Here is what Michelle had to say:
"Michelle Shullat 1:49 pm
Stuart, as best I can tell, you did everything right. I'm stumped. Would you mind opening a support ticket so we can give you a bit more individualized attention?"