Excellent point. Thanks again.
And an update on the RSS front, for interest’s sake: As an experiment, I tried creating a Feedburner feed from the API-associated Discourse feed, just to see if eliminating the API variable from the URL would make any difference with its processing. And sure enough, that version of the feed works fine with the WP plugin I’m using to access it. Sure seems to suggest that the issue was indeed with the plugin and its ability to interact with an API-involving feed following the certificate change.
Really appreciate all of the thoughtful pointers on this. I think I’m on the right path now to getting all of these lovely new quirks stamped out and solved!