I spent most of this evening going back and forth with support from the web host to figure out why my loadHTMLFile() calls weren't working.
Even though allow_url_fopen was enabled in the PHP settings the error logs showed the calls failing due to allow_url_fopen=0 despite making numerous configuration changes.
In the end I had to use cURL and convert the result to DOMDocument so I should be okay now. My testing looks good but I won't be completely certain until someone sends a webmention from their own domain.
In the meantime I added the missing items to the database so that the /directory OPML is up to date.