Simplest solution was to manually edit the relevant two records in forum_posts and forum_topics, keeping the original record IDs intact but changing all other variables. Then I simply deleted the second post. I previously copied the defunct OP into the new OP of the source.
After disabling our current test instance, assigning a spare domain to my new test instance, and updating my web server with said name string, the styling issues were resolved.
After further investigation and testing, I have found that my issues were caused by a URL string mismatch between my web server (nginx) and the URL defined in config_global.php. The web server was unable to cache resources as defined. Getting these synced properly was the solution, regardless of using domain name or raw IP.
Given this revelation, I have decommissioned my second test instance on the new server, and will spin that up again at my current test URL once I'm ready to deploy, decommissioning the first at that time.
No worries. This was resolved by rebuilding Achievements as you described
This is the solution (not your response above), so if you don't want to mark this post as a solution, you will need to repost it for posterity.