Update:
I did get a generic 404 page when clicking an internal URL on my development site but have no idea which error.php file is serving that. If I type in any silly page name, however, I get returned to the homepage. No 404 error page.
As above, please point me to the line in either error.php file where I can insert either the menu URL, the alias, or the article URL (and how that should be formatted).
Thanks again.